There are many books ______ that shelf. [ ]
A. on
B. in
C. at
答案：A
关于衰老的学说有（）
A.遗传程序学说
B.自由基学说
C.免疫学说
D.交联学说
E.端粒酶学说
侦察卫星的特点：速度快、效率高、效果好、适用广。
