Which of the following strings comply with the syntax rules and conventions for first$-$order formulas as specified in the prescribed book?

a$.$

$$x$($p$($x$)$yp$($x$,$y$))$

b$.$

$$x$($p$($x$)$q$($x$,$y$))$

c$.$

$$x$($p$($x$)$xq$($x$,$x$))$

d$.$

All of the above