—Can you do this Maths problem?
—Yes, I _____. [ ]
A. am
B. do
C. does
D. can
答案:D
Although he talked half an hour, he couldn't give us a good ________ of the reason why he did it.
A.idea
B.explanation
C.sense
D.understanding
在上地幔中,地震波速度由浅到深一直都是逐渐加快的。