题目

Growing up in Philadelphia, Lieberman started cooking with his stay-at-home dad when he was seven. His food-loving family had two kitchens, and he quickly learned what was the best way to bake his cakes. Lieberman improved his kitchen skills greatly during a year abroad before college, learning from a cook in Italy and studying local specialties(地方特色菜)in Germany, Spain and France. At Yale, he was known for throwing dinner parties, single-handedly frying and baking while mixing drinks for dozens of friends. Just for fun, he and some friends decided to tape a show named Campus Cuis点A是等边三角形BCD所在平面外一点,AB=AC=AD=BC=a,E、F分别在AB、CD上,且.设,表示EF与AC所成的角,表示EF与BD所成的角,则 [  ] A.f(λ)在(0,+∞)上是增函数 B.f(λ)在(0,+∞)上是减函数 C.f(λ)在(0,1)上是增函数,在(1,+∞)上是减函数 D.f(λ)在(0,+∞)上是常数
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