人工诱变培育新品种，目前最常用的方法是（）
A.用射线或激光照射
B.花药离体培养
C.秋水仙素处理萌发的种子
D.人工杂交
参考答案：A
However occupied you are with study, try to ______ at least 15 minutes each day to have some exercise.[ ]
A. put out
B. put through
C. put off
D. put aside
若要表达一个正弦交流信号必须具备三个要素，即（）。
A.振幅值
B.角频率
C.初相
D.相位
