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If something is obligatory, you must do it because of a rule or a law.
compulsory , necessary
optional , voluntary
Most women will be offered an ultrasound scan during pregnancy, although it's not obligatory.
大部分处于怀孕期间的妇女都将建议接受超声波检查，尽管这不是强制性的。
These rates do not include the charge for obligatory medical consultations.
这些收费不包括规定的医疗咨询费。
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