required
[rɪ'kwaɪəd]
adj.
必须的，（学科）必修的;
v.
要求( require的过去式和过去分词 );
需要;
想要;
命令;
例句:
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The three-piece percussion wing become quite forceful when required.
来自BBC
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Interest is paid monthly. Three months' notice is required for withdrawals.
利息按月支付。提款须提前3个月通知银行。
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The demand generated by one factory required the construction of another.
一家工厂产生的需求使得必须再建造一家工厂。
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Unions are required to give seven days' notice of industrial action.
工会组织劳工行动应提前7天作出声明。
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IMF backing was also required and was later given.
来自BBC
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To do this required a major orbital plane shift.
来自BBC