20.-----for a long time,but he tried his best to catch up with his classmates.A.Having been ill B.Being ill C.Tough he was ill D.He was ill

20.-----for a long time,but he tried his best to catch up with his classmates.A.Having been ill B.Being ill C.Tough he was ill D.He was ill
20.-----for a long time,but he tried his best to catch up with his classmates.
A.Having been ill B.Being ill C.Tough he was ill D.He was ill

20.-----for a long time,but he tried his best to catch up with his classmates.A.Having been ill B.Being ill C.Tough he was ill D.He was ill
D
but是连词,其前后两部分要对等.此例中but后面是一个分句,那么前面也应当是一个分句.如选A就不是分句了,就只是一个现在分词短语了,因为它没有主语,也没有谓语动词.现在分词是“非谓语动词.
同理,B也不能选.
but不与though连用,排除C.

后面有连词but,说明前后都是简单句

but是连词，连接的是两个句子，A的话前面不构成一个完整的句子。所以不选。