离心沉降速度的符号是（）。
A.ar
B.a
C.ut
D.u0
参考答案：D
一定条件下，可逆反应2A
B+3C，在下列四种状态中处于平衡的是（ ）
A．A
B．B
C．C
D．D
He is a teacher. He has been a teacher _____ 1990 and he _____ by his students.[ ]
A. in, is loved
B. until, loves
C. since, is loved
D. for, has loved
