人类已经在北极熊体内发现热带地区的农药残余,这种现象可能与( )①大气循环 ②水循环 ③生物循环 ④地壳物质循环相关
A:①②③④
B:①②④
C:②③④
D:①②③
— How terrible! We should try our best to protect the ___________.
____ are these postcards?
已知{an}是等差数列,且a2+a5+a8+a11=48,则a6+a7=( )