—Would you like me to help you with the maths problem?

—___________ It’s a most difficult one.

A．With pleasure.

B．Why not?

C．That’s all right.

D．It’s my pleasure.

对于粘弹性材料，若令应力保持一定，物体的应变随时间的增加而增大，这种现象称为（）

A、应力松驰

B、蠕变

C、滞后

D、虎克定律