Studying in groups is necessary __________ you want to do well in school.
A:if
B:until
C:unless
D:though
青春期的女生多吃水果不吃饭,既可以保证营养,又可以减肥.
在▱ABCD中,∠B﹣∠A=30°,则∠A,∠B,∠C,∠D的度数是( )