已知点(3,m)与点(n,-2)关于坐标系原点对称,则mn=______.
∵点(3,m)与点(n,-2)关于坐标系原点对称,
∴m=2 n=-3,
∴mn=-6.
保护生物多样性最为有效的措施是( )
A.建立濒危物种的种质库
B.颁布各种法律和文件
C.建立濒危物种繁育中心
D.建立自然保护区
Not until he went through real hardship _____the love we have for our families is important.
A.had he realized
B.did he realize
C.he realized
D.he had realized