Professor Shifty told her class that she would not help any students during normally scheduled office hours $($that she was bound by school rules to have$)$ unless she was paid $$25$ by each student before finals week and passed around a contract saying this. All students voluntarily signed. This is a$/$an

A$.$ valid contract because it has consideration, offer and acceptance.

B$.$ invalid contract because there was no consideration.

C$.$ invalid contract because there was no meeting of the minds, or mutual assent.

D$.$ valid contract because it was signed.