A light beam in air approaches a plane mirror with its direction adjusted so that it makes a 15∘ angle with the mirror’s normal. The mirror is then rotated by 5∘ toward the incoming beam (the normal rotates with the mirror). By how many degrees does the direction of the reflected beam rotate as a result of the mirror’s rotation?
(a) 20∘
(b) 5∘
(c) 10∘
(d) 12∘