根据语意的需要，“be+不定式”结构中不定式可以用被动式。如：
The cover is not to be removed. 这块盖布请勿揭开。
This medicine is to be taken hourly. 这药每小时服一次。
The cost of carriage is to be paid by the receiver. 运费由收货人支付。
She will determine how it is to be done. 她会决定这件事的做法。
A little tiredness after taking these drugs is to be expected. 服下这些药后会有些疲倦。
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