阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。
Why Does Rain Smell So Good?
Do you love the smell of rain? If so, you're not alone.
____(actual), some scientists believe that people got their love for the smell of rain from ____(ancestor), who relied on wet weather for their survival.
But what makes rain smell so nice? There are several smells associated with rainfall that people find pleasing.
One of these smells ____(call) “petrichor（尘土的味道）.” Petrichor, a term created in 1964 by two Australian scientists studying the smells of ____(rain) weather, comes from some chemical reactions.
During dry periods some plants produce oils, and when it rains, these oils go into the air. Another reaction occurs when chemicals ____(produce) by some bacteria are sent out. All of these chemicals combine to create the pleasant particular smell when rain hits ____ ground.
Another smell associated with rain is ozone（臭氧）. During a thunderstorm, lightning can split oxygen and another gad and ____(it) in turn can combine into a new substance. This substance reacts with other chemicals to form ozone, ____ has a sharp smell.
When someone says he can smell rain coming, it may be that wind from an ____(approach) storm has carried ozone down from the clouds and ____ the man's nose.
则
