Hi Yaesul,
The statement you mentioned -
current output... will at least be matched by additional yearly output from the remaining land - is perfectly constructed for us to use the Assumption Negation technique on.
Remember that if the question asks for an answer that must be true, or is required, or is one which the argument depends on, we can apply this technique. Since the conclusion requires the answer choice, the contra-positive tells us that if the answer choice is negated, then the conclusion must be false.
First negate this answer choice. Once you have this negated statement, look back at Betty's argument. Is her conclusion possible if we assume this negated answer?
Let us know if this helps or if you have further questions