Let f(t) be the probability density function for a life time of a hard drive, where t is measured in seconds. Explain in plain Enlish the meaning of each integral. (a) displaystyle integral from (36000)^(720000) f(t) dt (b) displaystyle integral from 0^(144000) f(t) dt (c) displaystyle integral from (365.25*24*60*60)^(infinity) f(t) dt (d) displaystyle integral from (0)^infinity f(t) dt