先行詞を含む関係代名詞について

おっしゃる通りです。このような関係代名詞は、日常的に使います。例えば、

Derek is 12. He is old enough to know what is right and what is wrong.
I have so many clothes. Tell me what is appropriate for a job interview.
This shelf shows what Regina gets by dancing. A lot of awards.
What you need is a vacation. Go to Florida and come back refreshed.

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