题干
Silk ______ one of the primary goods traded along the Silk Road by about 100 BC.
A：had become
B：was becoming
C：has become
D：is becoming
答案(点此获取答案解析）
A
Silk ______ one of the primary goods traded along the Silk Road by about 100 BC.
A：had become
B：was becoming
C：has become
D：is becoming
（i是虚数单位）的虚部为（ ）