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
网友回答
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。