No one tell us ＿＿¸so we need your help.宾语从句

题目

No one tell us ＿＿¸so we need your help.宾语从句
A how should we do.B what should we do C how to do it D what to do it
为什么选c how to do it 从句是提问什么?问什么要加介词?还有什么提问主语不用改变语序什么意思?

答案