[求助]lsat逻辑，请教(must be true)

Each December 31 in Country Q, a tally is made of the country's total available coal supplies - that is the total amount of coal that has been mined throughout the country but not consumed. In 1991 that amount was considereably lower than it had been in 1990. Furthermore Country Q has not imported or exported coal since 1990.

If the statements above ar true, which one of the following must also be true on the basis of them?

A In Country Q more coal was mined in 1990 than was mined in 1991. ( my answer )

B In Country Q, the amount of coal consumed in 1991 was greater than the amount of coal mined in 1991. ( correct answer)

我觉得题目里面没有讲过挖掘量跟消费量的关系，所以凡是选项提到消费的，我统统排除了。而且A选项正好是文章结论的重复，为什么不对呢？

或者是我对黄色部分理解不对？我的理解是：这种tally就是一个当年总的挖掘出但未消耗的部分。如果这样，文章也没说每交待挖掘数量相同啊？我就是想不明白。

1991年的TALLY=(1990的TALLY+1991开采的量-1991消费的量)<1990年的TALLY

说明"1991开采的量-1991消费的量"是一个负数,即消费的量大于开采的量.

原来1991年的tally数是一个累计数

万分感谢