There is a big tree _______ our classroom.
A:in front
B:in the front
C:in the front of
D:in front of
—Of course. Help____!
被称为地球之“肺”的是( )
若点B(a,0)在以点A(1,0)为圆心,以2为半径的圆内, 则a的取值范围为( )