若a是int型变量、且a的初值为6，则执行下面表达式后，a的值为（）。a+=a-=a*a
参考答案：60
Great changes ____________ in my hometown in the past few years.
A．took place
B．have taken place
C．have been happened
D．broke out
加氢反应器的类型有哪些？
