属于重体力活动水平的是（）。
A.学生日常活动
B.售货员
C.办公室工作
D.舞蹈
E.机动车驾驶员
参考答案：D
O2在血液中运输的主要形式是( )。
A．物理溶解
B．高铁血红蛋白
C．氨基甲酸血红蛋白
D．氧合血红蛋白
E．化学溶解
Somebody rang my doorbell just now, but I didn't know _____. [ ]
A. who were they
B. who they were
C. who it was
