若a•23=26,则a等于( )
A:2
B:4
C:6
D:8
农业区位选择的实质是( )
—This history book is of great value.
—Exactly. _____ can be enjoyed from it ______ you have a deep understanding of it, however.
自然界的变化可以分为____变化和____变化两类。