A

flex: 1 == flex: 1 0 0px.

B

Setting flex-grow/flex-shrink to different values can make items grow/shrink relative to each other

C

But only if flex basis is 0! If they are all 0px in width, then all the width on the line is extra. flex: 5 item gets 5 times as much width as the others a flex: 1.