When I came back up , I heard enthusiastic WEE - OO - WEE - OO coming from his room . I popped my head in there and he made the noise at me and grinned . I asked " Firetruck ? "

Question with options to choose from: Why might he make that sound ?
pick from the following.
(a). I said " firetrucks " so he made the noise ..
(b). He was playing make - believe with himself ..
(c). I like firetrucks so he made the noise ..
(d). He felt enthusiastic that I was in his room ..
(b).