Yahoo! joined the AdMarket 50 stock index last week as its newest component. The Internet company takes the slot formerly held by CBS Corp., which ceased trading as an independent company following its acquisition by Viacom. Viacom remains part of the AdMarket 50 index. Yahoo! joins two other major Internet companies in the index: America Online and Internet services company MarchFirst, created by the merger of USWeb/CKS and Whittman-Hart. In trading Friday, Yahoo! closed at $125.38, down 3.74% for the week.