When a child was born, her grandfather decides to put $500 in an account that earns interest. He plans to make no other deposits or withdrawals for 18 years. When the child turns 18 years old, the money in the account will be a birthday gift. The grandfather is choosing between two options: Option A: An account that grows by 10.5% each year. Option B: An account that grows by $20 each year. Which option will result in a better 18th birthday gift? Explain your reasoning.