Two drivers drink the same number of standard drinks over the same evening and end up with noticeably different blood alcohol concentrations. Which factor best explains the difference?

Why D is correct

Blood alcohol concentration is a ratio: alcohol divided by the body water it can dissolve into. Two people of different composition hold different amounts of that water, so the same number of drinks produces different concentrations, and average physiological differences between the sexes shift the ratio again. Food matters because alcohol leaves a stomach slowly when there is something else to work on, spreading absorption across more hours. Fatigue, medication and illness add further variation. The practical consequence is that counting drinks tells a driver almost nothing reliable about their own level.

Why the other options are wrong

A. Nothing explains it — the count of drinks settles the concentration

Counting is a starting estimate at best. Because the denominator differs from person to person, one count maps onto a range of results wide enough to straddle the legal line.

B. Only a difference in personal tolerance can explain it

Tolerance explains why someone feels less affected, not why the measured concentration differs. A tolerant driver is often in a worse position: they feel fine at a level already impairing them.

C. The type of drink matters most, since spirits absorb faster than beer

Standard drinks are defined to be equivalent in alcohol content. Mixing spirits changes the volume in the glass, not the equivalence the standard unit was created to express.

On the road

Two colleagues have two beers each at the same event. One is smaller, drank on an empty stomach, and feels wrong behind the wheel; the other feels untouched. Neither can tell from how they feel which side of the line they are on.

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