题目

I arrived in the classroom，ready to share my knowledge and experience with 75 students who would be my English literature class．Having taught in  1  for 17 years，I had no   2  about my ability to hold their attention and to  3  on them my admiration for the；literature of my mother tongue．     I was shocked when the monitor shouted，“  4  !”The entire class rose as I entered the room，and I was somewhat   5  about how to get them to sit down again，but once that awkwardness was over，I quickly   6  calmness and began what I thought was a fact—packed lecture已知铯137发生衰变的方程是，其半衰期约为30年。以下说法正确的是 [     ]A．式中的来自于铯的核外电子 B．经过60年后，受铯137污染的土壤将仍含铯137C．降低温度可以有效地减小铯137的衰变速度 D．铯137发生衰变时的质量亏损即等于β粒子的质量