The word "sustainable" in the last paragraph is closest in meaning to__________.阅读材料，回答下面的题目。 A New Strategy to Overcome Breast CancerPost-menopausal ( 绝经后 ) women who walk for an hour a day can cut their chance of breast cancer significantly,a study has suggested. The report, which followed 73,000 women for 17 years, found walking for at least seven hours a week lowered the risk of the disease. The American Cancer Society team said this was the first time reduced risk was specifically linked to walking. UK experts said it was more evidence that lifestyle influenced cancer risk. A recent poll for the charity Ramblers a quarter of adults walk for no more than an hour a week, but being active is known to reduce the risk of a number of cancers. This study, published in Cancer Epidemiology,Biomarkers&Prevention, followed 73.615 women out of 97,785 aged 50-74 who had been recruited by the American Cancer Society between 1992 and 1993,so it could monitor the incidence of cancer in the group. They were asked to complete questionnaires on their health and on how much time they were active and participating in activities such as walking, swimming and aerobics ( 有氧运动) and how much time they spent sitting watching television or reading. They completed the same questionnaires at two-year intervals between 1997 and 2009. Of the women, 47% said walking was their only recreational activity. Those who walked for at least seven hours per week had a 14% lower risk of breast cancer compared to those who walked three or fewer hours per week. Dr. Alpa Patel, a senior epidemiologist at the American Cancer Society in Atlanta, Georgia, who led the study, said: "Given that more than 60% of women report some daily walking, promoting walking as a healthy leisure-time activity could be an effective strategy for increasing physical activity amongst post-menopausal women. We were pleased to find that without any other recreational activity, just walking one hour a day was associated with a lower risk of breast cancer in these women." "More strenuous ( 紧张的) and longeractivities lowered the risk even more." Baroness Delyth Morgan, chief executive of Breast Cancer Campaign, said: "This study adds further evidence that our lifestyle choices can play a part in influencing the risk of breast cancer and even small changes incorporate into our normal day-to-day activity can make a difference."She added: "We know that the best weapon to overcoming breast cancer is the ability to stop it occurring in the first place. The challenge now is how we turn these findings into action and identify other sustainable lifestyle changes that will help us prevent breast cancer."All of the following factors relating to cancer risk were mentioned in the EXCEPT__________.

A. affordable
B. available
C. persistent
D. continual