switch-case statement without break

According to this book I am reading：

Q What happens if I omit a break in a switch-case statement?

A The break statement enables program execution to exit the switch construct. Without it, execution continues evaluating the following case statements.

Suppose if I have codes looking like

switch (option}{
    case 1:
    do A;
    case 2:
    do B;
    default:
    do C;
    break;
}

Does this mean if I choose case 1, the A and C are done. If I choose case 2, B and C are done. If i choose neither, then only C is done.

if so, what happens if we omit the break after do C.

I assume these are bad programming practice, but I am curious what would happen to get a deeper understanding how it all works. Thanks