麦类、稻类、谷子每茎正常籽粒（）粒为有效茎。
A.≥6
B.≥4
C.≥3
D.≥5
参考答案：D
老鹰和麻雀在空中飞行．如果它们的动能相等，则老鹰的速度较______；如果它们的重力势能相等，则麻雀所处的位置高度较______．
Mr Smith________his living by teaching ten years ago and he________greatly from his work. [ ]
A. earned; earned
B. earned; gained
C. gained; gained
D. gained; earned
