函数f（x）=$\left\{\begin{array}{c}\sin x,\sin x\le \cos x\\ \cos x,\sin x>\cos x\end{array}\right.$，下列四个命题

①f（x）是以π为周期的函数

②f（x）的图象关于直线x=$\frac{5\mathrm{\pi }}{4}$+2kπ，（k∈Z）对称

③当且仅当x=π+kπ（k∈Z），f（x）取得最小值﹣1

④当且仅当2kπ＜x＜$\frac{\mathrm{\pi }}{2}$+2kπ，（k∈Z）时，0＜f（x）≤$\frac{\sqrt{2}}{2}$

正确的是____

把下列词语补充完整。

风____浪____            ____ ____飞瀑

____头____进            ____ ____罕至

    One day, Allan and his friend Henry went swimming in a river. It was very hot. How happy they were in the river! After they got out of the water, they played games in the sun for a while.
    On their way back, Henry saw some flowers. He liked flowers very much and ran into the green field to look at them. Now Allan was walking by himself.
    Then he heard Henry calling out, "A snake（蛇）! Help!..."
    "What's wrong with you?" asked Allan.
    "A snake bit（咬）me in the leg. Come here!"
    Allan ran over and saw a small red wound（伤口）on Henry' s leg.
    "The snake was in the grass. I didn't see it."
    "Sit down quickly"! Allan told Henry.
    Allan put his mouth at the little red wound and began to suck（吸）at it. In this way he saved （挽救）Henry's life. "Oh, Allan, it's very kind of you to help me."
    "That's all right. We are friends and we must always help each other."