The Search Time for a Particular Search Term on Google

Multiple Choice

The search time for a particular search term on Google is defined by the probability density function below, where x = random variable "time to produce a result in milliseconds".Determine the standard deviation of x. 1/5 for values of x between 250 and 255
f(x) =
0 everywhere else

A) 2.083
B) 3.001
C) 1.673
D) 2.298
E) 1.443