So difficult_____it to acquire a large vocabulary without the help of a teacher that he was close to giving up .

A．he have felt

B．have he felt

C．he did feel

D．did he feel

重量分析中作初步洗涤时，对晶形沉淀，可用（）进行洗涤，由于（）效应，可以减少沉淀的溶解损失，但如果沉淀剂为不挥发的物质，就可改为（）等其它合适的溶液洗涤沉淀.