A reason or a cause

I don’t see any … why you can’t come with us. 

The … for his success was because of hard work. 

Carelessness is often the … of fires.

Speeding is often … of car accidents. 

What was the … of the explosion?

The police was known a … of an accident. 

The … of the accident is still not known. 

Scientists attribute changes in the weather to natural … 

Never stay away from school without good … 

He is the … of all our troubles.

The … of the accident was the ice on the road.

The …. why the house was destroyed by the storm was because the foundations were very poor.