上个世纪90年代后期，许多欧洲城市面临着由于汽车造成的空气和噪音污染日益严重的状况，

以下各选项中能正确声明一个表示50个值为null的字符串数组的是( )。

A．string[]a；

B．string a[]；

C．chara[50][]

D．stringa[]=newString[50]

概括下面这则新闻的主要内容，不超过25个字。

中国绕月探测工程总指挥栾恩杰接受了新华社记者的采访，称“绕月工程不是‘天价’”。绕月探测工程从研制到建设，花费了14亿元，不是“天价”。

14亿元，大概是l4公里高速公路的造价。如果修地铁的话，可能也只能修两三公里。可我们修了38万公里从中国到月球的路，是一条真正的“天路”。   

整个工程的研制、建设，六千多人的队伍，二百多家单位，三年多的时间就完成了，真正体现了“少花钱、办大事”的鲜明特色。

工程花费不多跟我们的指导思想是分不开的，那就是充分利用成熟的技术，充分利用现有的装备和设施来开展。火箭、地面的设施、卫星平台和技术等，都是适应嫦娥一号卫星的要求做一部分修改。

已知平面图形的形心为C，面积为A，对Z轴的惯性矩为Iz，则图形对Z1轴的惯性矩为（）。

A.A

B.B

C.C

D.D

The movie Batman and Joker is ______ one that I've ever seen. [ ]

A. more exciting

B. more excited

C. the most exciting

D. the most excited

Every year, 2,000 American lives are saved by the selflessness of others. These are the bone marrow donors who give the gift of life to patients fighting deadly diseases such as leukemia, lymphoma, and aplastic anemia. That’s the good news. The bad news is that thousands more die each year because not enough people have signed on to the registries that would help the ill find a suitable match for a transplant.
Bone marrow or stem-cell transplants are usually a last resort, intended for those whose illnesses have not responded to traditional treatments such as chemotherapy or radiation. How do they work We all store a special type of cell in our bone marrow called stem cells. These primitive cells give rise to the three types of blood cells: red, white and platelets. Everyone’s stem cells have certain genetic characteristics or markers that make them unique from others. Despite this uniqueness, there are some shared characteristics between people. This is important, because a patient’s immune system will reject blood or organs received from someone else if they do not share sufficient similarities.
Family members, especially siblings, are always the first to be considered as donors, because there’s a greater chance that the genetic markers on their cells will have enough in common to prevent rejection after transplantation. In many cases, however, a familial match can’t be found and then the search begins for an unrelated donor. These donors typically come from a pool of people who have already signed up on a donor registry in the event that their cells match a needy recipient.
Once the lab has verified a match between donor and recipient, the next phase starts. The patient is given radiation or chemotherapy to kill the unhealthy cells. Healthy cells are harvested from the donor — either extracted from the pelvic bones or taken from the arm in a way that is similar to having blood drawn — and prepared in a laboratory. Once they’re ready, they’re given to the patient through a vein — the same way as one would receive a blood transfusion. Once these transplanted donor cells get settled within the patient’s bone marrow, they make the healthy red blood cells, white blood cells and platelets necessary to support life.
One of the major problems currently faced by transplant centers is that while bonemarrow transplants can cure more than 70 different diseases, there aren’t enough donors on the registry to treat the more than 3,000 patients awaiting transplants. The National Marrow Donor Program, the largest registry in the country, has approximately 4.8 million adult volunteer donors, but that isn’t nearly enough for the thousands who need transplants.
Why isn’t a pool of more than 4 million donors isn’t enough to cover 3,000 needy patients Here’s the reason: in the vast-majority of cases, finding a suitable match isn’t easy. Because we are unique individuals with a variety of ancestral backgrounds and integration patterns, finding someone similar to us is a major task. In the end, it comes down to a numbers game — the more potential donors listed on the registry, the greater a chance of finding a match, especially for those with unusual genetic characteristics.

Who is considered the best donor for bone marrow transplant

A．Parents.

B．Brothers and sisters.

C．Registered donors.

D．Healthy donors.