To show that here exist reserves for which vickrey auction obtains 2-approximation to the optimal auction single-item. Why should the fact that the value distributions are independent be used?

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3.3 The Vickrey auction with reserve prices R=(r1,,rn), sells an item to the bidder i with bid biri that has the highest bid, if such a bidder exists. The payment that this winning bidder makes is the maximum of their reservation price ri and the bid of the highest other bidder j that bids at least their reservation price, rj. Show that whenever value distributions are independent, there exist reserves R=(r1,,rn) for which Vickrey auction obtains 2 of the revenue of the optimal auction single-item auction