--- -How do you improve your writing skills? ________. A.I have trouble in reading B. It’s a bit difficultC.I like the movie best D. By finding a pen pal 答案:D具有A、B、C三种性质的总体,其容量为63,将A、B、C三种性质的个体按1:2:4的比例进行分层抽样调查.如果抽取的样本容量为21,则A、B、C三种性质的个体应分别抽取( )A.12,6,3B.12,3,6C.3,6,12D.3,12,6