题目

（共5小题；每小题2 分，满分10 分）       根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。选项中有两项为多余选项。     The right pair of exercise shoes can do a lot to prevent discomfort and injuries, personal trainers say. Here are some shopping tips:   71   Depending on your workout, shoes designed for walking, running, tennis, or something else that will provide the best support for your feet. Shop when you normally exercise. Feet actually get bigger throughout the day, sometimes swelling(肿胀) up as much as a half若（1+x）n=1+ax+bx2+…+cxn-1+xn，n∈N，且b：c=2，那么n=    ．