大约在80个表现正常的人中有一个白化病致病基因杂合子。一个表现型正常，其双亲也正常但

党和国家领导人多次强调：“我国必须坚持人民民主专政”。这是因为[ ]

①人民民主专政是民主与专政的统一 ②人民民主专政的本质是人民当家作主 ③坚持人民民主专政作为四项基本原则之一，是我国的立国之本 ④坚持人民民主专政是社会主义现代化建设的政治保证

A．①②

B．①④

C．②③

D．③④

I was a single parent of four small children, working at a low paid job. Money was always tight, but we had a (1) over our heads, food on the table, clothes on our backs, and if not a lot, always (2) . Not knowing we were poor, my kids just thought I was (3) . I’ve always been glad about that.

It was Christmas time, and although there wasn’t (4) for a lot of gifts, we planned to celebrate with a family party. But the big (5) for the kids was the fun of Christmas (6) .

They planned weeks ahead of time, asking (7) what they wanted for Christmas. Fortunately, I had saved MYM120 for (8) to share by all five of us.

The big (9) arrived. I gave each kid a twenty-dollar bill and (10) them to look for gifts of about four dollars each. Then everyone scattered. We had two hours to shop; then we would (11) back at the "Santa’s Workshop"

Driving home, everyone was in high Christmas spirits, (12) my younger daughter, Ginger, who was unusually (13) She had only one small, flat bag with a few candies—fiftycent candies I was so angry, but I didn’t say anything (14) we got home. I called her into my bedroom and closed the door, (15) to be angry again. This is what she told me.

"I was looking (16) thinking of what to buy, and I (17) to read the little cards on the ’Giving Trees’. One was for a little girl, four years old, and all she (18) for Christmas was a doll. So I took the card off the tree and (19) the doll for her. We have so much and she doesn’t have anything. "

I never felt so (20) as I did that day.

2().

A.wanted 

B.did 

C.got 

D.played

小车升降台定位是靠升降台下边的（）来定位的

A.接近开关

B.光电开关

C.行程开关

什么叫循环倍率？为什么要保持锅炉一定的循环倍率？