If a physician hears two "lub" sounds instead of one, then which of the following conditions is true?

A) The semilunar valves are not closing simultaneously.
B) The atrial blood is flowing backward and causing the extra sound.
C) The atrioventricular valves are not closing at the same time.
D) The AV and semilunar valves are not closing at the same time.
E) No such double sound has ever been heard.