Which statement describes the relationship between a recurrent neural network

$($RNN$)$ and a sequence$-$to$-$sequence $($Seq$2$Seq$)$ model?

Seq$2$Seq models run on RNNs

A Seq $2$ Seq model is a combination of multiple RNNs$.$

A Seq$2$Seq model is an RNN with an additional attention layer.

RNNs and Seq$2$Seq models do not have any relationship between them.