【单选题】
How many broadcast domains are shown in the graphic assuming only the default vlan is configured on the switches?()<br /> <img src="https://nimg.ppkao.com/2018-07/zhanglijun/2018072317285690760.jpg?sign=91e5f51bc341e0218e0f810b6e751dfc&t=62d44060" /><br />
A、one
B、six
C、twelve
D、two