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∵S5=S10,∴S10-S5=a6+a7+a8+a9+a10=0,根據等差數列的性質可得,a8=0∵等差數列{an}遞減,∴d<0,即a7>0,a9<0,根據數列的和的性質可知S7=S8為Sn最大.故選D.
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