()5. There is often fog spring mornings.

A.in
B.at
C.on

C

时间介词用法：at用于具体时刻前；in用于年、月、泛指的早中晚等场景前；当morning这类表示早/中/晚的名词带有限定修饰词，指代特定的某段早中晚时，要使用介词on。本题中spring mornings是带有修饰限定的特定早晨，因此选on。

二、填写合适的单词补全短文。

Hiking in the mountains can be lots of 1. , but there are some important things we should know.
First, we can't go hiking in bad weather. It's very dangerous. We have to check the weather report before we go. And it's often windy in the mountains, so we should be careful of 2. .
Second, we should bring enough 3. water and food on our trip. In the mountains there is often little clean water and food.
Third, it's important to wear hiking shoes and 4. clothes. The sun is often strong in the mountains, so we had better wear sunglasses to protect our eyes.
Last but not least, we should tell others before we go, like our 5. or friends. They need to know where we are.
Have a great time, but please stay safe.

1. fun 2. fires 3. drinking 4. comfortable 5. parents

我们结合备选词汇和上下文语境逐一分析：
1. 第一空：固定搭配lots of fun表示“很多乐趣”，符合山间徒步的属性，选fun。
2. 第二空：前文提到山里经常刮风，刮风容易引发火情，所以要小心火灾fires，符合安全提示的语境。
3. 第三空：固定搭配drinking water表示“饮用水”，符合徒步出行要带足量饮用水和食物的语义，选drinking。
4. 第四空：此处需要形容词修饰clothes，comfortable意为“舒适的”，符合徒步穿搭的合理要求。
5. 第五空：和后文的friends并列，是出行前需要告知行程的对象，parents（父母）符合逻辑。