Which of the following agreements is unenforceable because of lack of valid consideration? Johnson agreed to donate $1,000 to the United Hospital Building Fund. On the basis of this and similar promises, construction was started. Davis promised to give his neighbor's son $50 if the boy would refrain from trespassing on his property. Because of Davis' promise, the boy complied. Boyd agreed to sell his used motorboat to Yates for $500. Yates offered the $500. Philips promised his nephew $200 if the boy would refrain from smoking until he reached age twenty-one. Because of this promise, his nephew did not smoke.