Mules are considered to have many superior characteristics compared to the horse or burro. Mules are often stronger, agile, and can jump higher than either of their parents. Why are mules considered to be a weak hybrid and not a separate species?

Because they are sterile

Explanation:

Mules are considered to be a weak hybrid and not a separate species because they are sterile and cannot produce progeny of their own.

Mules are hybrid products from two separate species - a donkey and a horse.

A species is generally defined as a group of organisms that are capable of breeding to produce fertile offspring. Hence, in order for them to have the potential of being categorized as a separate species, they must be able to produce their own progeny but this is not the case.

your answer is "the 1950's"

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.