单选题Many cast iron pistons are designed with heat dams, which serve to （）Akeep piston crown temperatures elevated for smoother combustionBreduce the possibility of overheating the top compression ringChelp retain the heat of compression to prevent ignition delayDhelp retain the heat of compression to prevent combustion knock
单选题
Many cast iron pistons are designed with heat dams, which serve to （）
A
keep piston crown temperatures elevated for smoother combustion
B
reduce the possibility of overheating the top compression ring
C
help retain the heat of compression to prevent ignition delay
D
help retain the heat of compression to prevent combustion knock
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共用题干第三篇Iron DeficienciesExperts have observed for years that endurance athletes,particularly females,frequently have iron deficiencies.Now a new study by a team of Purdue University researchers suggests that even moderate exercise may lead to reduced iron in the blood of women."We found that women who were normally inactive and then started a program of moderate exercise showed evidence of iron loss,"says Roseanne M.Lyle,associate professor at Purdue.Her study of 62 formerly inactive women who began exercising three times a week for six months was published in the journal Medicine & Science in Sports & Exercise."Women who consumed additional meat or took iron supplements were able to bounce back,"she notes."But the new exercisers who followed their normal diet showed a significant decrease in iron levels."Iron deficiency is very common among women in general,affecting one in four female teenagers and one in five women aged 18 to 45,respectively.But the ratio is even greater among active women,affecting up to 80 percent of female endurance athletes.This means,Lyle says,that"too many women ignore the amount of iron they take in."Women of childbearing age are at greatest risk,since their monthly bleeding is a major source of iron loss.Plus,many health-conscious women increase their risk by rejecting red meat,which contains the most easily absorbed form of iron.And because women often restrict their diet in an effort to control weight,they may not consume enough iron-rich food,and are liable to experience a deficiency."The average woman takes in only two thirds of the recommended daily allowance of iron," notes another expert."For a woman who already has a poor iron status,any additional iron loss from exercise may be enough to tip her over the edge into a more serious deficiency,"notes the expert. Exercise can result in iron loss through a variety of mechanisms.Some iron is lost in sweat,and,for unknown reasons,intense endurance exercise is sometimes associated with bleeding of the digestive system.The best sources of iron,and the only sources of the form of iron most readily absorbed by the body,are meat,chicken,and fish.Good sources of other forms of iron include dates,beans,and some leafy green vegetables.Who will have a significant decrease in iron levels?A:All the women.B:All the inactive women.C:Women who start to do moderate exercise and follow their normal diet.D:Women who start to do moderate exercise and consume additional meat or take iron supplements.
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