Which of the following is the correct rule for the sequence 1, 2, 2, 4, 8?

_{
}a_{1}=1, a_{2}=2, a_{n}=a_{n-1}-a_{n-2}

_{
}a_{1}=1, a_{2}=2, a_{n}=a_{n-1}+a_{n-2}

_{
}a_{1}=1, a_{2}=2, a_{n}=2a_{n-1} a_{n-2}

_{
}a_{1}=1, a_{2}=2, a_{n}=a_{n-1} a_{n-2}