单模光纤，其纤芯直径约为（）um，包层直径为125um.
A.4~10
B.2~20
C.10~20
D.50
参考答案：A
波峰焊焊接工艺中，预热工序的作用是（）
I was surprised to see（）at the concert.
A．those number of people
B．that amount of people
C．that number of people
D．those amount of people
