I really don’t know ___ ___I kept thesecret papers.

30How long do you think it will be _____ we can find a new water source?

下列各组词语中加点的字注音无误的一组是(　　)

The maths problem was so difficult thatnone of us could ____.

We all felt a senseof ______ when we saw so many overseas Chinese coming back safeand sound from Libya