浇筑混凝土前清理灰尘和垃圾时发尽量使用()。
A.吹风机
B.吸尘器
C.冲洗机
D.扫帚
参考答案：B
已知等差数列{an}中，a3=30，a9=90，则该数列的首项为______．
--l'm worried, Jim. I don't know how ________ writing.
--Don't worry! Why not ________ a pen pal? [ ]
A. learn; to find
B. learn; find
C. to learn; find
D. to learn; to find
