A frequency distribution of the number of books read by students in a month is given in discontinuous form: 0–4 (frequency 5), 5–14 (frequency 18) and 15–24 (frequency 12). After converting the classes to continuous form, what will be the frequency density of the second class?
(a) 1.6
(b) 1.8
(c) 3.6
(d) 2.0