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A certain store sells two types of pens: one type for $2 per pen and the other type for $3 per pen. If a customer can spend up to $25 to buy pens at the store and there is no sales tax, what is the greatest number of pens the customer can buy?
A 9
B 10
C 11
D 12
E 20
这个题首先就可以想到单价越便宜的我就能买的越多，因为它没有说两种笔一定要都买。