The art club is for members only. You can't go in _______ you are a member.
A:unless
B:because
C:if
D:though
如图,在梯形ABCD中,AD∥BC,∠B=45°,∠C=120°,AD= 436 ,BC=4 2 ,求CD的长.
经济重心南移是中国古代重要的历史现象.关于这一现象的表述,符合史实的是( )