下列属于我国必须坚持的发展战略是()①可持续发展 ②计划生育 ③节约资源 ④环境保护 ⑤人才强国 ⑥科教兴国
A:①②③
B:②③④
C:④⑤⑥
D:①⑤⑥
We should never give up ___________ others. Sometimes helping others means helping ourselves.
如图,在正三棱柱ABC﹣A′B′C′中,若AA′=2AB,则异面直线AB′与BC′所成角的余弦值为( )