A language school has found that its students can memorize (p(t) = 60t) phrases in (t) hours of class (for (1 ≤ t ≤ 10)). Find the instantaneous rate of change of this quantity after 4 hours of class.
a) (60) phrases/hour
b) (120) phrases/hour
c) (180) phrases/hour
d) (240) phrases/hour