当观众看到足球运动员即将进球是，就会心跳加速，血压升高而显得面红耳赤，这主要是什么物质作用的结果（ ）

    Hi, I'm John. There are four seasons in a year. I like them all. In spring, it's warm. There are pretty flowers everywhere. I often go on a picnic with my parents. Summer is hot. I often go swimming in the pool. I like to wear my sunglasses. In fall, the weather is cool. I can help my grandpa pick apples. Winter is white. I can make a snowman. And I can go skating, too.