三个连续的自然数，中间的数是x，则三个数的和是______．
（x-1）+x+（x+1）=3x，
答：则三个数的和是 3x．
故答案为：3x．
Those are _____________.[ ]
A. their cloth
B. their clothes
C. his ruler
D. her ruler
简述障景的作用？
