—I see you got a "D" in biology．_         ?

—Well, I found that subject hard． I think I'II drop it for history．

A．How about that

B．What for

C．How come

D．What's your next plan

一列长210m的列车，在进入510m的隧道口时鸣笛，站在隧道另一端的某人听到笛声38.5s后，看到火车全部驶出隧道，若火车的行驶速度为18m/s且保持不变。问:这时的声速是多少？