Peter bought $2$ watermelons, a banana and an apple, which cost a total of $20$ dollars. Joey bought a banana and $2$ apples, which cost $7$ dollars less than Peter. The unit price of watermelon is $3$ dollars higher than that of banana. How much is an apple? 
选项为:A:$$3$$ dollars  B:$4$ dollars  C:$5$ dollars  D:$6$ dollars  E:$$7$$ dollars  请回答其中一个选项.