Quiz: Clause Conversion

You want to express "$x_1$ must be true" as a $2$-SAT clause. How do you write it?

1. A.$(x_1 \lor x_1)$
2. B.$(x_1 \lor \neg x_1)$
3. C.$(\neg x_1 \lor \neg x_1)$
4. D.You cannot express single-variable constraints in $2$-SAT