Which of the following answers is the best answer regarding access control. a$.$ Access control to tenant's data can only be done by the cloud. b$.$ Access control to tenant's data cannot be done by in an untrusted cloud. c$.$ In a trusted cloud, access control can be done using the Open Authentication $($OAuth$)$ protocol. d$.$ Group$-$level access control in an untrusted cloud can be done using proxy re$-$encryption.