$1.$ In a PROFESSOR and COURSE relationship if the PROFESSOR object is given a cardinality of $(1,1),$ then which of the following would be a true statement about the university?

a$.$ a PROFESSOR must have more than one COURSE to teach

b$.$ a PROFESSOR can optionally teach a COURSE

c$.$ a COURSE must have at least one PROFESSOR to teach

d$.$ a COURSE can optionally have a PROFESSOR to teach