$(5$ pts$)4.$ A small speaker emits sound waves uniformly in all directions. Point $P$ is $20\mathrm{.}0$ m from the speaker. By what multiplicative factor must the power output of the speaker be increased in order to increase the sound intensity level at point $P$ by $20$ dB $?$

$($a$)10$

$($b$)20$

$($c$)100$

$($d$)200$

$($e$)400$

$($f$)$ none of the above answers