Autonomous System $($AS$)$: Consider the network shown below. Suppose AS$3$ and AS$2$ are running OSPF for their intra$-$AS routing protocol. Suppose eBGP and iBGP are used for the inter$-$AS routing protocol. Initially suppose there is no physical link between AS$2$ and AS$4.$ Router $3$b learns about x from which routing protocol: OSPF, eBGP, or iBGP?