A capacitor is charged up to $18$ V $,$ and then connected across a resistor. After $10$ seconds, the capacitor voltage has fallen to $12$ V $.$ What will the voltage be after another $10$ seconds $($that is$,20$ seconds total$)?$

a$)0$

d$)9$ V

b$)6$ V

e$)10$ V

c$)8$ V