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

a$)$ "The user has peid the subserption fec, but does not enter a valid password."

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

c$)$ "Access is denied if the user thas not paid the subscription $(cec{)}^n$

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