-----Mr. Smith, can I come to ask for some advice on my paper at any time?

-----Yes, of course.             .

A．If you’re convenient

B．At your convenience

C．That’s right

D．Don’t mention it

线路允许速度160km/h≥vmax＞120km/h，波浪型磨耗谷深超过（）mm时为轻伤钢轨。

A、0.3

B、0.5

C、1

D、2