任何实验都具有实验者、（）和实验对象3个基本因素。
A.实验手段
B.实验目的
C.实验步骤
D.实验工具
参考答案：A
_______ in the 1800s and first mass-produced in Germany, the harmonica(口琴) was designed to play simple folk tunes.
A．Being invented
B．Inventing
C．Invented
D．Having invented
Look. This is _____ clock. It's _____ alarm clock. [ ]
A. a; an
B. an; an
C. an; a
D. a; a
