若m，n是不等于1的正整数，且log3m•log3n≥4，则m+n的最小值是（ ）
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∵m，n是不等于1的正整数，
∴log3m＞0，log3n＞0
∴log3m+log3n≥2log3m•log3n
∵log3m•log3n≥4，
∴log3m+log3n≥4
∴mn≥81
∴m+n≥2
=18（当且仅当m=n时取等号）mn
∴m+n的最小值是18
故选C．
The Slender West Lake Scenic Spot lies in the northwestern part of Yangzhou, with its south gate facing Great Rainbow Bridge Road. It covers an area of about 123.6 hectares. The Slender West Lake was named a National Key Scenic Spot in 1988 and National AAAAA Tourist Area in 2010.  The Slender West Lake used to be a long river named the Baozhang Lake, which is 4 kilometers in length and less than 100 meters in width. A famous poet Wang Hang in the Qing Dynasty once composed a poem (诗歌), from which the Slender West Lake gets its present name and to which it owes its great fame (名声) both at home and abroad. During the Sui and Tang Dynasties, many gardens were built along the Slender West Lake. Following the lines of the natural landscape, a large number of buildings along the moats(护城河) were constructed(建造) in different locations during different periods. Emperors Kangxi and Qianlong of the Qing Dynasty both visited the Slender West Lake. The Slender West Lake Scenic Spot is an outstanding representative (代表) of the lake gardens in China. In a word, the Slender West Lake not only represents the beauty of natural landscape（风景）, but also embodies (体现) the richness of its history and culture. The Slender West Lake
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