HNCO可用于消除汽车尾气中的NO和NO₂，其反应原理为：HNCO + NO_x → N₂ + CO₂ + H₂O，下列说法正确的是（   ）

泡菜酸辣脆嫩，很多人都喜欢自己制作泡菜食用。制作泡菜时要用特殊的坛子，坛口要加水密封。密封坛口的主要目的是（ ）。

设N_A为阿伏伽德罗常数的数值，下列叙述中正确的是（　　）

解方程或比例。 

阅读理解

    What is the first thing you want to do when you see someone yawn? You want to yawn, too! Yawning is contagious.
    Scientists have found that all people yawn. Babies start to yawn even before they are born. The earliest yawn takes place when a baby is still inside its mother. When young children are about one year old, they start to yawn when they see others yawning. That's why scientists think yawning is contagious.
    It's no secret that yawning is contagious, but why do we yawn? Well, scientists are not really sure.
    Some think we yawn because we feel bored. Others believe yawning has something to do with breathing. When there is too much carbon dioxide(CO₂) in our bodies, our bodies will tell the brains to breathe deeply. That causes a big yawn.
    In 2007, scientists said people yawned to cool down their brains. These scientists found people yawned more often if they were warmer than others.
    Scientists are still trying to find out why yawning happens and why it is so contagious. But they are only sure that when one person yawns, immediately everyone else nearby wants to yawn, too.