a and b
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その二つの案は甲乙つけがたい。
It is hard to say which of the two proposals is better.
品質に甲乙はあるが、どちらも使える。
There are differences in quality, but both can be used.
経験だけでは甲乙を決められない場合もある。
Sometimes experience alone is not enough to determine which is better.