在以下位字段结构的定义中，不正确的是(   )
A、struct test
{
    unsigned a:1;
    unsigned b:2;    
    unsigned c:17;   
}
B、struct test
{
    unsigned a:1;
    unsigned b:2;    
    unsigned c:12;   
}
C、struct test
{
    unsigned a:1;
    unsigned b:2;    
    unsigned c:7;   
}
D、struct test
{
    unsigned a:1;
    unsigned : 0;    
    unsigned c: 7;   
}