Is a type of carbohydrate plants use to add strength and support to their cell walls.

The answer to this question would be: Cellulose

Cellulose is a carbohydrate with the long chain that goes from hundreds into thousands link. The long chain makes cellulose has a high tensile strength so it was good to support the plant. 
A human can't digest cellulose that is why we can't eat a tree. But some herbivore like termite have enzyme that will be able to digest cellulose

If the water vapor content stays the same and the temperature drops, then the relative humidity increases. its vise versa.if the water vapor content stats the same and the temperature rises, the relative humidity decreases. so, yes, i'd say that temperature and humidity have a strong relationship.