(a) If the distance from a point sound source triples, the sound intensity will be (1) 3,...

Question:

(a) If the distance from a point sound source triples, the sound intensity will be (1) 3, (2) 1/3, (3) 9, (4) 1/9 times the original value. Why?
(b) By how much must the distance from a point source be increased to reduce the sound intensity by half?