Which command revokes ACL-based write access for groups and named users on the file afile?

Which command revokes ACL-based write access for groups and named users on the file afile?
A . setfacl Cx group: * : rx, user:*: rx afile
B . setfacl Cx mask: : rx afile
C . setfacl ~m mask: : rx afile
D . setfacl ~m group: * : rx, user :*: rx afile

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Answer: C