Multi-choice problem: Continue writing the next sentence in this paragraph:

A man is riding a rowing machine in a gym. a finger

pick from the following.
(a). appears in front of him and he turns to find out why.;
(b). briefly covers part of the camera lens.;
(c). moves the levers in front of the machine.;
(d). taps on the windshield of the machine.;
(b).