Which of the Following Is a Valid Contract Under UCC

Which of the following is a valid contract under UCC?

A) Alex offers to sell his house to Billy for $100,000 and gives a written promise. On the 29th day, Billy calls up Alex to accept the offer.
B) Alex offers to sell his house to Billy for $100,000 and gives a written promise to keep the offer open for 10 days. Billy calls up Alex on the fifth day accepting the offer and on the 10th day goes to buy the car. Alex asks for $155,000.
C) Alex offers to sell his house to Billy for $100,000 and gives a written promise to keep the offer open for 10 days. Two days later, Alex sends a telegram to Billy to revoke the offer. On the morning of the fifth day Billy receives Alex's telegram. Billy calls up Alex the same afternoon accepting the offer.
D) Alex offers to sell his house to Billy for $100,000 and gives a written promise to keep the offer open for 10 days. Billy offers to pay him $1,000 extra for keeping the offer open for 20 days. Two days later, Billy calls Alex to revoke the offer. Alex calls up Billy on the ninth day to accept the offer.
E) Alex offers to sell his house to Billy for $100,000 and gives a written promise to keep the offer open for 10 days. On the third day Billy calls up Alex, declining the offer. On the tenth day, Alex asks Billy to come and buy the car at the agreed price.