分解因式：3mn2-12m=______．
3mn2-12m，
=3m（n2-4），
=3m（n+2）（n-2）．
What would have happened ________，as far as up to the river bank.
A．Bob had walked farther
B．if Bob should walk farther
C．had Bob walked farther
D．if Bob walked farther
种群的集群现象有何生态学意义？
