Cho phản ứng: NaCrO₂ + Br₂ + NaOH →  Na₂CrO₄ + NaBr + H₂O. Khi cân bằng phản ứng trên, hệ số của NaCrO₂ là

A. 1.  

B. 2

C. 3

D. 4

- You 'd better not eat too much meat.

- He drivers carefully.

- It takes me six hour to drive from London to Edinburgh.

- My sister drinks less coffee than my brother.

- Before I went to school, I went over my lesson.

- My aunt started teaching English 40 years ago.

- After I had had a cup of coffee, I came to class.

- It is three months since he lost the job.

- Jonh has been playing the organ for five years.