【简答题】鱼类的皮肤有什么特点?有何机能?其主要的衍生物有哪些?
【单选题】They often help ______ sick children in the children’s hospital. A. cheer up B. clean up C. set up
【单选题】Most children need encouragement in time of failure ______ they can cheer up again.
【单选题】两个用丝线悬吊的通草球,一个带电,另一个不带电,将两球靠近时,不可能发生的现象是( )
【单选题】Most children need encouragement in time of failure _____ they can cheer up again.
【单选题】一个带电的通草球靠近另一个不带电的通草球,两球将会( )