1 . 必要
Nnecessary必要negated conjunction 否连言
2 . 必备
Extensive use of aircraft , plastics recycling is necessarymachinery manufacturers ., 该机用途广泛 , 是塑料再生厂家必备机械.
3 . 必需的
...hould)+v.。类似的形容词还有:important(重要的), essential(必要的), necessary(必需的), advisable (明智的), desirable (所希望的), vital (生死攸关的
4 . 必要时
Where a duty payer considers it necessary, it may offer to file an additional declaration ., 纳税义务人认为必要时 , 也可以主动要求进行补充申报.