Forward Declarations

Consider a header file Foo. $h$ that declares, but does not define, a class type $U.$ Which of the following would prevent the use of a forward declaration of type T in Foo. $h?$ Select all that apply.

Pick ONE OR MORE options

T is a base class of $U$

U has a $($public or private$)$ member variable whose type is T&

U has a $($public or private$)$ member variable whose type is $T^{*}$

U has a $($public or private$)$ member variable whose type is $T$

U defines a nested type $U$::$V$ as an alias for $T,$ but the alias is not referenced elsewhere in the header file