$2-23$ A cellular operator needs a database to keep track of its customers, their subscription plans, and the handsets $($mobile phones$)$ that they are using. The E$-$R diagram in Figure $2-24$ illustrates the key entities of interest to the operator and the relationships between them. Based on the figure, answer the following questions and explain the rationale for your response. For each question, identify the element$($s$)$ in the E$-$R diagram that you used to determine your answer.