At a certain symphonic concert, tickets for the orchestra level were $50 and tickets for the balcony level were $30. These two ticket types were the only source of revenue for this concert. If R% of the revenue for the concert was from the sale of balcony tickets, and B% of the tickets sold were balcony tickets, then which of the following expresses B in terms of R?
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A$${200R}\over{500+3R}$$
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B$${300R}\over{500-2R}$$
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C$${300R}\over{200+5R}$$
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D$${500R}\over{300+2R}$$
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E$${500R}\over{200+3R}$$
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正确答案: D