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A is corrent because unrecorded sales would decrease both sales and gross profit by equal amounts and thus decrease the ratio. Note that the ratio is less than 1.0.
B is incorrect because fictitious sales would increase both sales and gross profit by equal amounts. The result would be an increase in the ratio. Note the ratio is less than 1.0.
C is incorrect because merchandise purchases being charged to general expense would increase gross profit while sales are unaffected. The result would be an increase in the ratio.
D is incorrect because unrecorded purchases would increase gross profit while sales are unaffected. The result would be as an increase in the ratio.