In running time analysis, which of following is not one of the reasons why we focus on the worst$-$case

running time?

$($a$)$ The average case time is often difficult to determine.

$($b$)$ The worst case is easier to analyze.

$($c$)$ It is crucial to applications such as games, finance and robotics.

$($d$)$ The running time of an algorithm inversely grows with the input size.