be able to 和can的区别

都表示能,能够,但是因为主句是过去时,从句也应该用过去时,can 和be able to都有过去时的形式,即could 和was able to ,所以只能选择D.
b更强调因为能力而成功完成.
另外,can 和be able to有区别主要是：
1．can只有“现在式和过去式”；
I can swim.
He could run after the bus ancatch catch it twenty years ago.
be able to除了有“现在式和过去式”外,还有“未来和完成”的概念.
Can you ride on a bicycle
I shall not be able to come again today.
I shall have been albe to do something.
2．过去式的“could”表示过去的“能力”、“潜力”和“可能性”.如：
Lanny could run and was a good tennis player
I could run after a bus and catch it twenty years ago
过去式的“was/were was/were able ”不能单纯的表示过去的“能力”、“潜力”和“可能性”；只能表示“能力 + 成功的完成一个动作”而且更加侧重强调后者.