the bargaining power

Which one is correct?
a) the power of customers (the bargaining power of customers) is high because of the existence of the providers' switching costs. (i think it's false)
b) the power of customers is very high because the supermarkets are the main client. (i tried looking for statistics in internet... but no luck... where do people most buy beer? bars, supermarkets...)
c) the power of customers is low. However, the supermarkets are the main customer due to the supermarkets' trademark value.
d) the power of customers is relatively low due to the customers' fragmentation. (maybe true)

ONLY ONE TRUE STATEMENT . THANK YOU VERY, VERY, VERY MUCH FOR YOUR HELP!!! BEST REGARDS, SARAH ([email protected]) if you dont mind, i need to know how you got the answers: references, statistics... because i need to be sure they are accurate. Thank you so much!!!!