The following data have been reported for wettability studies on oil$/$brine$/$rock system by contact angle method.

${}_{ow}=30$dyne$\frac{s}{c}m,{}_{so}=50$dyne$\frac{s}{c}m,{}_{sw}=40$dyne$\frac{s}{c}m.$

a$)$ What is the contact angle for the solid$/$oil$/$water system?

b$)$ What is the wettability of the system $($oil$-$wet or water$-$wet$)?$

On the next experiment, if ${}_{so}=70$ dynes $\frac{?}{cm}$ is reported instead,

c$)$ What is the contact angle for the solid$/$oil$/$water system?

d$)$ What is the wettability of the system $($oil$-$wet or water$-$wet$)?$