按线圈的结构分类，变压器有哪几种？
参考答案：
4种：①单绕组变压器②双绕组变压器③三绕组变压器④多绕组变压器
_____ a US visitor visa twice, the young man finally gave up his hope of going to America. [ ]
A. Having been refused
B. Being refused
C. Having refused
D. To be refused
