Express these system specifications using the propositions $p$ : "The user enters a valid password," $q$ : "Access is granted," and $r$ : "The user has paid the subscription fee" and logical connectives $($including negations$).$

a$)$ "The user has paid the subscription fee, but does not ent$$r a valid password."

b$)$ "Access is granted whenever the user has paid the subscription fee and enters a valid password."

c$)$ "Access is denied if the user has not paid the subscription fee."

d$)"$If the user has not entered a valid password but has paid the subscription fee, then access is granted."