下面各组数中，（ ）组的第二个数是第一个数的因数． A．1.5和3 B．7.2和

函数y=ax+1在R上是单调递减的，则函数g（x）=a（x²-4x+3）的增区间是（　　）

A．[2，+∞）

B．[-2，+∞）

C．（-∞，2]

D．（-∞，-2]

黄金是一种贵重金属。在人类漫长的历史中，素有“沙里淘金”、“真金不怕火炼"之说。这说明黄金在自然界中主要以       形式存在，也说明黄金的化学性质在自然条件下非常      

Today I’d like to talk about how to become a (1) . Usually you have to start at the beginning. That is, you have to begin (2) of some sort on a local newspaper, a (3) , maybe on a small magazine -- work (4) . Now there aren’t that many organizations which have large numbers of foreign correspondents so you also have to (5) with that aim (6) . Another way of doing it is to work abroad, and work as (7) . That’s somebody who sells stories, and gets paid (8) , to newspapers and magazines and so on. And hoping to (9) that way, and eventually to be offered a (10) .
I don’t really know (11) are needed, but obviously a degree in modern languages or (12) would be very useful. But, a degree in itself probably wouldn’t (13) . It wouldn’t be enough to get you job. It wouldn’t (14) to make an editor decide to (15) . So it’s difficult to define the (16) . You’re going to need (17) and the ability to assimilate information (18) and then produce stories which (19) of the newspaper or radio or television station you (20) .

信号在随参信道中传输时，产生频率弥散的主要原因是（）。