A:study
B:little
C:about
I was told to finish the work in three ______time.
甲、乙两名同学在参加体育中考前各作了5次投掷实心球的测试,甲、乙所测得的成绩的平均数相同,且甲、乙成绩的方差分别为0.62、0.72,那么( )
一棵杨树的生命系统结构层次为( )