量块按“级”使用时，是以量块的实际中心长度尺寸作为工作尺寸。( )
参考答案：错
古希腊数学家把数1，3，6，10，15，21，…叫做三角形数，它有一定的规律性．若把第一个三角形数记为a1，第二个三角形数记为a2，…，第n个三角形数记为an，计算a2-a1，a3-a2，a4-a3，…，由此推算，可知a100=______．
The number of people invited _____ fifty, but a number of them _____ absent for different reasons. [ ]
A. were; was
B. was; was
C. was; were
D. were; were
