$14-$Which of the following statements about UML$2$ sequence diagrams are true?

a$)$ A sequence diagram shows the communication and interactions between a system and its external environment.

b$)$ A synchronous message is shown by an open arrowhead

c$)$ You cannot refer to other sequence diagrams

d$)$ Objects and actors involved are listed along the left of the diagram, wih a dotted line drawn horizontally from them.