Two of the largest portals for Internet advertising are _________ and __________; both try to filter out illegitimate clicks but advertisers on the sites still claim to be victims of click fraud.

Question

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Google and Yahoo! are two of the largest portals for Internet advertising and both try to filter out illegitimate clicks, but advertisers on the sites still claim to be victims of click fraud.

Two of the Largest Portals for Internet Advertising Are _________