After returning from summer vacation, phil notices that his teenage neighbor, alexander, has begun to shave and his voice had deepened. phil concludes that these changes are most likely due to

I believe the answer is: puberty

Puberty highlighted the period of time when people reach our sexual maturity. (the time when we able to reproduce). During this time, most people would undergine our most intense growth spurt, which might also cause us develop in other areas of our body (adams' apple, breast, height, etc)

imperial crisis leads to civil war.

roman forces withdraw from britain as goths attack rome.

odoacer topples romulus augustus

huns attack the empire and drive german peoples ahead of them.