【判断题】标准,按执行力度可分为强制性标准和推荐性标准。
【单选题】He has some trouble ____ his classmates but he has no trouble ____ doing his lessons. [ ]
【单选题】―What your English teacher doing now? ―He _____ a newspaper.
【单选题】叶酸缺乏可使胎儿神经管畸形的发病率增高,因此建议妊娠前和妊娠早期每日补充叶酸为多少维克
【简答题】发动机的磨合规范由( )( )和( )组成。