(单选题)3: 对于任意两个随机变量X和Y，若E（XY）=E（X）E（Y），则有（ ）。 A: X和Y独立 B: X和Y不独立 C: D（X+Y）=D（X）+D（Y） D: D（XY）=D（X）D（Y） ...

(单选题)17: 对于任意两个随机变量X和Y，若E（XY）=E（X）E（Y），则有（ ）。 A: X和Y独立 B: X和Y不独立 C: D（X+Y）=D（X）+D（Y） D: D（XY）=D（X）D（Y） ...

(单选题)12: Jack’s mother has to (A) call him at least half a dozen (B)times until(C) he comes down(D) to his dinner.（选择有错误的一项） A: has to B: half a dozen C: until D: comes down ...

(单选题)25: A ___ to this problem is expected to be found before long. A: result B: response C: settlement D: solution ...

(单选题)19: I finally ___ the reference in a dictionary of quotations. A: track out B: track down C: track up D: trace down ...

(单选题)17: It is high time that such practices ___. A: are ended B: be ended C: were ended D: must be ended ...

(单选题)5: It was essential that the application forms ___ back before the deadline. A: must be sent B: would be sent C: be sent D: were sent ...

(单选题)19: It's bad ________ for you to smoke in the public places where smoking is not allowed. A: behavior B: action C: manner D: movement ...

(单选题)7: I don't know the park, but it's ________ to be quite beautiful. A: said B: told C: spoken D: talked ...

(单选题)1: It is important that the hotel receptionist ___ that guests are registered correctly. A: make sure B: has made sure C: made sure D: must make sure ...