— I have got a backache.

— That’s not surprising. You ______ in front of that computer too long.

A．work

B．worked

C．have been working

D．are working

如图试管内水蒸气将塞子推出的过程，水蒸气的       能转化为塞子的机械能。

四冲程汽油机工作过程中的        冲程与上述能量转化过程相同。