For an ad entering an auction on the Display Network, the Google Ads system will calculate the effective cost–per–thousand impressions (eCPM) when there are:
only cost-per-click (CPC) ads entering the auction
both cost-per-click (CPC) and cost-per-acquisition (CPA) ads entering the auction
only viewable cost-per-thousand impressions (vCPM) ads entering the auction
any cost-per-click (CPC) or cost-per-acquisition (CPA) ads entering the auction – CORRECT