For each variable, determine whether it is best thought of as discrete or continuous.

$\backslash$table$[[$Variable$,$Discrete,Continuous$],[($a$)$ The length of the arm of a heavyweight boxer,$0,0],[\backslash$table$[[($b$)$ In an experimental study, the participant's estimate for the hei$],[$projected $12$ meters away$]],0,0],[($c$)$ The number of defective compact discs in a batch of $500$ discs,$0,0],[($d$)$ The cranial capacity $($space inside the skull$)$ of a human skull,$0,0]]$