—They ______ really hard. There is no success without effort.
A:studied
B:enjoyed
C:compared
D:practiced
如图是光学显微镜的4个镜头,甲乙一端无螺纹,丙丁有螺纹.若要在视野内看到最多的细胞,宜选用的镜头组合是( )
已知:如图,在△ABC中,AD平分外角∠EAC,∠B=∠C.求证:AD∥BC.
要说明命题:“一组对边平行且对角线相等的四边形是矩形”是假命题,可以举的反例是( )