What is nullable type

Question

which of the following are tuue about Nullable type?
A- a Nullable type is refrence type a refrence type.
B- an implicit  conversion exists from any non-nullable value type to a nullable type t? exists if there is a predefined  conversion from
C-a predefined conversion  from the nullable type s? to the nullable  type t? exist if there is a predefined  conversion from the non-nullable type s to the non nu