—What do you usually do ________ your parents aren't at home?
—I do my homework. Sometimes I watch TV or play computer games.
A:because
B:while
C:but
D:before
在生物圈中有“绿色水库”之称的生态系统是( )
下列各组词语中,没有错别字的一组是( )
4个平面最多可将空间分割成____ 个部分.