We    on it for several hours but we haven't yet reached any conclusion.

A．are working

B．work

C．have been worked

D．have been working

车体钢结构、各部钢板裂纹、腐蚀深度超过（）破损时焊修、挖补或补强。

A.45%

B.50%

C.55%

D.60%