Sorry,for my misunderstanding and misleading advice.
I thought B says " been improving a lot." in the Perfect continous tense.But it isn't.
In this case,my judgement should be changed like this:
1.Both B & D are correct.
2.In terms of the best choice,it should be D.
Because in most cases,we don't use the passive voice when we say,sth gets better = improve.
sth + be improved,the passive voice,is also possible,when you especially emphasizes the passive action,which sound a little bit unnatural.