问题
选择题
I must have been a great _____ to my parents in those hard days, for I never failed to live up to their
expectations.[ ]
A. achievement
B. comfort
C. disappointment
D. support
答案
答案：B
搜题请搜索小程序“爱下问搜题”
衰变，则该核发生
衰变时的衰变方程：
______(衰变后的元素用Y表示）；若其半衰期为8天，则经过______天有75%的X核发生了衰变。