No one has yet succeeded in explaining the ________ of how life began.
A. puzzle
B. cause
C. problem
D. reason
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You must get there within an hour. There should be no ___ in sending this information to him.
A. question
B. problem
C. quarrel
D. delay
I can't give you the card without Smith's ________.
A. agreement
B. allowing
C. permission
D. perfomance
What impressed me most was that they never lost ________.
A. heart
B. hearts
C. their heart
D. their hearts
Though I spoke to him many times, he never took any ________ of what I said.
A. remark
B. observation
C. attention
D. notice