紧急求助.. 二元一次方程问题.....

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Marta bought several pencils.  If each pencil was either a 23-cent pencil or a 21-cent pencil, how many 23-cent pencils did Marta buy?

(1)  Marta bought a total of 6 pencils.

(2)  The total value of the pencils Marta bought was 130 cents.

答案是B...没有问题..

但是我的问题是...你如何知道B 只有唯一解...

23X+21y=130....

因为有的时候GMAT这类题..有是是唯一解, 有时是和A组成方程组球解...总不能考试的时候一个个试吧!!!!!.....因为错了好几次这类的题,都是因为不知道什么时候是唯一解...决的因为这样错题..也太冤枉了.....求NN帮助帮助....