One can figure out the meaning of “airmail” to be “mail by air”

One can figure out the meaning of “airmail” to be “mail by air” by its （ ）.
A、onomatopoeic motivation
B、morphological motivation
C、semantic motivation
D、etymological motivation
【正确答案】：B
【题目解析】：根据这个词的形态就可以猜测出它的大概意思。