Answer:
A. Concentration gradient must exist.
Step-by-step explanation:
Umm.......I think I knw this answer but it will take me a quite bit of time so I will answer it for u tmrw if u don't mind huh? :)
Poppy
a=((b1+b2)/2)h
a/h=(b1+b2)/2
2a/h=b1+b2
2a/h-b1=b2