今有质量分数为20%的某溶液一瓶,倒出3/4质量后加水至原质量,再倒出2/3质量后,余下溶液的质量分数为[ ]
A、5%
B、6%
C、4%
D、3%
答案:A
These pictures _______ when we went to Mountain Tai last weekend.[ ]
A. took
B. were taken
C. will take
D. will be taken
双闭环直流调速系统起动过程的特点是()、()以及()。