1、John said he’d been working in the office for an hour,-was

1、John said he'd been working in the office for an hour,＿was true.
A、he B、this C、which D、who
2、＿in thought,he sat still in the sofa.
A、losing B、having lost C、lost D、to lose
3、The research is so designed that once＿nothing can be done to change it.
A、begins B、having began C、beginning D、begun
希望能得到比较详细的答案
题一中B为什么不行？
题二中B为什么不行？
题三中B为什么不行？

CCA
由于是逗号,是一句话不是两句.只能填which不能真this.故选C
lost指He was lost in thought.简化为lost in thought故选C
once 后面承前省了主语.省掉了The research ...
应为once (the research) begins 故选A

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