听力原文： Between 1987 and 1991 ,three groups of American women, numbering 27 in all, between the ages of 35 and 65,were given month-long tests to determine how they would respond to conditions resembling those aboard the space shuttle. Though carefully selected from among many applicants, the women were volunteers and pay was barely above the mini mum wage. They were not allowed to smoke or drink alcohol during the tests, and they were expected to tolerate each other’s company at close quarters for the entire period. Among other things, they had to stand pressure three times the force of gravity and carry out both physical and mental tasks while exhausted from strenuous physical exercise. At the end of ten days, they had to spend further twenty days absolutely confined to bed, during which time they suffered backaches and other discomforts, and when they were finally allowed up, the more physically active women were especially subject to pains due to a slight calcium loss. Results of the tests suggest that women will have significant advantages over men in space. They need less food and less oxygen and they stand up to radiation better. Men's advantages in terms of strength and stamina, meanwhile, are virtually wiped 'out by the zero-gravity condition in space. The average number of women in each group is ______.