The difference between the maximum and minimum values of the function $sinx=$bcosx is

$($a$)2\sqrt[\phantom{2}]{a^2+b^2}$

$($b$)2(a^2+b^2)$

$($c$)\sqrt[\phantom{2}]{a^2+b^2}$

$($d$)-\sqrt[\phantom{2}]{a^2+b^2}$