题目内容：

    Athletes who can run several marathons in just months might seem unstoppable.The biggest obstacle，it turns out，is their own bodies.A new study find out for the first time a “ceiling” for endurance activities such as long­distance running and biking.

“Physiologists and athletes alike have long been interested in just how far the human body can push itself.When exercising over a few hours，a wealth of evidence suggests most people max out at about five times their basal metabolic(代谢的)rate(BMR)．How humans use energy during longer endurance activities is another question entirely，”says Herman Pontzer，an evolutionary theorist at Duke University，North Carolina.

Pontzer saw an opportunity to answer that question when Bryce Carlson，an endurance athlete and former biochemist at Purdue University，organized the Race Across the USA in 2015.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.”

1.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.

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

A.The hydrogen and oxygen in their drinking water.

B.The carbon dioxide their bodies create.

C.The amount of their urine，sweat and breath.

D.The remaining isotopes in their body.

3.What is the purpose of Pontzer's study?

A.To confirm that an athlete's physical extreme is mainly determined by his BMR.

B.To compare human limits in common exercise with those in endurance activities.

C.To find out where the highest energy consumption of an athlete lies.

D.To set a reasonable standard for researches concerned with human body

4.How did Pontzer feel when their study results came out?

A.Excited. B.Regretful.

C.Surprised. D.Hopeful.