绘图铅笔上的H和B分别表示、，H前数字越大表示铅笔芯越，B前数字越大表示铅笔芯越。
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I was doing ____secretarial work.
I think the ____thing would be to take a taxi home.
Lighting levels should be ____for photography without flash.
Some sign languages are very ____means of communication.
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