Find the reference number for each value of $t$.
(a) If $t=\frac{2\pi}{3}$,
the reference number is $\bar t=$ ;
(b) If $t=\frac{3\pi}{4}$,
the reference number is $\bar t=$ ;
(c) If $t=-\frac{2\pi}{3}$,
the reference number is $\bar t=$ ;
(d) If $t=\frac{13\pi}{4}$,
the reference number is $\bar t=$ ;
(e) If $t=\frac{7\pi}{6}$,
the reference number is $\bar t=$ ;
(f) If $t=-\frac{11\pi}{3}$,
the reference number is $\bar t=$ ;