如果光照不足，而温度偏高，这时叶片的CO2补偿点（）。
A．升高
B．降低
C．不变化
D．变化无规律
参考答案：A
Not once ________ to Michael that he could one day become a top student in his class.
A．occurred it
B．it did occur
C．it occurred
D．did it occur
已知月球绕地球做匀速圆周运动的周期为T、轨道半径为r，地球表面的重力加速度g，试求出地球的密度．（引力常量G为已知量）
