A student differentiates p(x)=3x−4cosx and writes p′(x)=3+4sinx. Which one of the following statements correctly describes the student's result?
a) p′(x) should be 3−4sinx; the student missed the negative sign.
b) The student’s result is correct.
c) p′(x) should be 3−4cosx; the student differentiated cosx incorrectly.
d) p′(x) should be 3x+4sinx; the derivative of 3x was taken incorrectly.