The morpheme of plurality /-s/ is pronounced as /s/ in cats, /z/

The morpheme of plurality /-s/ is pronounced as /s/ in cats, /z/ in bags, and /iz/ in matches. This example shows___________.
A、morphemes are abstract units
B、allomorphs are phonetically conditioned and thus predictable
C、morphs are actual spoken, minimal carriers of meaning
D、monomorphemic words coincide with words and function freely in a sentence
【正确答案】：B
【题目解析】：根据题干，复数形式的发音体现的是词素变体，