Susan sells 20 gasoline futures contracts at the futures price of $0.545 per gallon. Size of each contract is 42,000 gallons. The initial margin is $1,200 per contract, and the maintenance margin is $800 per contract. At the close of trading on the first day, the futures price rises to $0.555 per gallon. Determine the amount of variation margin required at the end of the first trading day.

a) $15,600

b) $8,400

c) $0

d) $420

e) $400