Which is considered a boundary between two different fronts?

cold front

neither warm front nor cold front

both warm front and cold front

warm front

Both warm front and cold front is considered a boundary between two different fronts.

Option C

Explanation:

When the cold front and warm front come into play, they give rise to a weather front. The cold front is formed from the edge of cold air mass whereas warm front from the vice versa.

When both the cool and hot air meet each other the boundary that gets created between is the weather front. In other words, the temperature and humidity of the masses are different which results in the weather front. Different weathers get formed as a result of such fronts.

81 is a composite number.