Answer:
unenforceable because Agro's performance was preexisting duty.
Explanation:
In the situation being described, it can be said that their agreement is unenforceable because Agro's performance was preexisting duty. This refers to the party's offer of a performance that was already required of them under the existing contract making a modification null. In this scenario, this is exactly what is happening, Agro Excavations has already signed a contract to dig the pond and has no enforceable reason to add $15,000 to the contract price mid-project and must finish digging the pond for the agreed-upon price of the first contract.