In the spoken English of some areas in the US, the "r" sound

In the spoken English of some areas in the US, the "r" sounds at the end of the words
22. In the spoken English of some areas in the US, the "r" sounds at the end of the
words .
A. are dropped B. drop
C. are being dropped D. have dropped
知道答案选A,问题的the "r" sounds 为什么用复数,r 有2个以上的发音吗?