Runners covered 4，957 kilometers over the course of 20 weeks in a series of marathons stretching from Los Angeles，California，to Washington，D.C.

To find out how many calories the athletes in the study burned，Pontzer，Carlson，and colleagues replaced the normal hydrogen and oxygen in their drinking water with harmless，uncommon isotopes（同位素）of those elements. By chemically tracing how these isotopes flush out in urine，sweat，and breath，scientists can calculate how much carbon dioxide an athlete produces — a measure that directly relates to how many calories they burn.

Pontzer's team then collected energy consumption data over the course of the race to see how many calories they burned per day.They found that whatever the event is，energy consumption leveled off after about 20 days，eventually staying at a steady level at about 2.5 times an athlete's BMR.At that point，the body is burning calories more quickly than it can absorb food and turn it into energy，representing a biologically determined ceiling on human performance.

“It was just one of those moments of discovery that as a scientist you just live for，” Pontzer said.“We ended up plotting out the very limits of human endurance，the envelope for what humans can do.”

29．What does the underlined word “ceiling” in the first paragraph refer to？

A．The weakness of human bodies.

B．The effect of lasting exercise.

C．The way humans use energy.

D．The limit of human endurance.

30．Which of the following is a direct measure of an athlete's energy consumption？

A．The hydrogen and oxygen in their drinking water.