Amonohybrid cross is made between plants that

have experienced only one hybrid generation
differ in only one trait
are identical
are self-pollinated

SOS:

The answer is differ in only one trait!

The definition of monohybrid cross is "The breeding of two organisms which differ in a single trait."

Hope this helps!

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.