Answer each of the following questions 'yes' or 'no', and justify your answer. (a) Suppose f(x) is an increasing function of one variable. Is f(x) quasiconcave?

(b) Suppose f(x) is a decreasing function of one variable. Is f(x)quasiconcave?

(c) Suppose f(x) is a function of one variable and there is a real number b such that f(x) is decreasing on the interval (,b] and increasing on [b,+). Is f(x) quasiconcave?

(d) Suppose f(x) is a function of one variable and there is a real number b such that f(x) is increasing on the interval (,b] and decreasing on [b,+). Is f(x) quasiconcave?