(Write down the measurement unit when applicable and correct your answers to 4 decimal places.) According to the guideline from Dental Association, a dentist can use up to a maximum of 5 times of nitrous oxide ("laughing gas") in a single treatment. Let X be the number of times of nitrous oxide being used by a dentist in a single treatment. The probability distribution of X is as follows:

x 0 1 2 3 4 5

P(X = x) 0.01 0.08 0.23 0.34 0.26 0.08

(a) Find the expectation of X. (2 marks)

(b) Find the variance of X. (2 marks)

(c) A researcher is reviewing reports of 4 independent treatments conducted by this dentist. What is the probability that among the 4 treatments, exactly 3 of them have used nitrous oxide for 4 or 5 times? (6 marks) (Total: 10 marks)