单选题：QuesUonsare based on the following paassageTrying to quanti

QuesUonsare based on the following paassage
Trying to quantify your aerobic fitness (有氧适能)is difficult since it usually requires access to an exercise-physiology lab.But researchers at the Norwegian University of Science and Technology have developed a low-tech means of precisely assessing aerobic fitness and estimating your "fitness age", or how well your body functions physically, relative to how well it should work, given your age．
The researchers evaluated almost 5,000 Norwegians between the ages of 20 and 90, using mobile labs．They took about a dozen measurements, including height, body mass index, resting heart rate, HDL (高密度脂蛋白)and total cholesterol (胆固醇)levels． Each person also filled out a lifestyle questionnaire．Finally, each volunteer ran to the point of exhaustion on a treadmill (跑步机)to measure his or her peak oxygen intake (VO2 max) 一an indicator of fitness age, sine VO2 max is found to closely correlate with significantly increased life spans, even among the elderly or overweight．
In order to figure out how to estimate VO2 max without a treadmill, the scientists examined the results to determine which of the data points were most useful． Surprisingly, they found that putting just five measurements--waist circumference (腰围); resting heart rate; frequency and intensity of exercise; age; and sex-r-into an formula allowed them to predict a person's VO2 max with great accuracy, according to their study published in Medicine & Science in Sports & Exercise．
The researchers used the data set to make a table of the typical, desirable VO2 max for a healthy person at every age from 20 to 90, creating specific parameters (参数)for fitness age． The concept is simple enough, explains Ulrik Wisloff, a professor at the Norwegian University and the senior author of the study．"A 70-year-old person who has the peak oxygen intake of a 20-year-old has a fitness age of 20," he says．He has seen just this combination during his research．
The researchers have used all of this data to create an online calculator that allows people to determine their VO2 max without going to a lab. Just plug the numbers of the five measurements into the calculator, and you'll learn your fitness age． To determine your resting heart rate, sit quietly for 10 minutes and check your pulse; count for 30 seconds, double the number and you have your resting heart rate．
The results can be alarming． A 50-year-old man, for instance, may have a fitness age of 59． Thankfully, unwanted fitness years can be erased. Exercise more frequently or more intensely． Then replug your numbers and your "age" will decline． A youthful fitness age, Dr． Wisloff says, "is the single best predictor of current and future health．"
What can we infer about aerobic fitness? A．It's impossible to measure for the need of special labs．
B．It's difficult to measure precisely．
C．It can be quantified by your fitness age．
D．It shows how well your body should function．