Let f$($x$,$y$,$z$)=0\mathrm{.}78$x$+6\mathrm{.}23$y$3\mathrm{.}39$z$4\mathrm{.}11.$ Using the slope of each variable, complete the sentences below.

a$)$ The curve f Select an answer increases decreases by units for every $1$ unit increase in x$.$

b$)$ The curve f Select an answer increases decreases by units for every $1$ unit increase in y$.$

c$)$ The curve f Select an answer increases decreases by units for every $1$ unit increase in z$.$