【正确答案】In sentence a, we only know Jack was on his bed, and his action is not sitting but lying. What Jack was doing while he was lying is not mentioned in sentence a. In sentence b, we know Jack was sleeping in his bed.

【答案解析】[解析] 本题型旨在考查考生对句意的理解，句子意义是由组成句子的词语意义和句子的结构意义相互作用下整合出来的。