What is the key difference between value types and object types in Qt$?$

a$.$

Value types can emit signals, while object types cannot.

b$.$

Object types provide advanced features like signal$-$slot communication and introspection, while value types do not.

c$.$

Value types are stored on the heap, while object types are stored on the stack.

d$.$

Object types cannot be passed between functions, while value types can.