To see a young couple loving each other is ；but to see an old couple loving each other is the best sight of all.
A. no wonder
B. no doubt
C. no way
D. no problem
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(答案→)A. no wonder
B. no doubt
C. no way
D. no problem
解析：考查固定词组，A. no wonder不足为奇B. no doubt无疑D. no problem没问题，句子意思“将要看见年轻的一对相亲相爱不足为奇，但是看见一对老人相亲相爱是最美丽的风景”