Because natural disasters occur at random times it is difficult to warn people .A. at pre

问题补充：

Because natural disasters occur at random times, it is difficult to warn people .

A. at present

B. at last

C. on time

D. in advance

答案：

A. at present

B. at last

C. on time

(答案→)D. in advance

解析：at present目前，现在；at last最后；on time准时；in advance事先，预先。句意是因为自然灾害是随时发生的，很难预先警告人们。故选D。