31. It won’t make much _______ whether you agree or not.

A. difficulty

B. trouble

C. difference

D. matter

32. No one has yet succeeded in explaining the ______ of how life began.

A. cause

B. problem

C. reason

D. puzzle

33. What impressed me most was that they never lost ______.

A. hearts

B. heart

C. their heart

D. their hearts

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