下列各题运算正确的是[ ]
A．3x+3y=6xy
B．x+x=x2
C．﹣9y2+16y2=7
D．9a2b﹣9a2b=0
答案：D
It was about 600 years ago __________ the first clock was made.
A．that
B．until
C．before
D．when
水谷之海(太仓)
