4. [50 Points] A company database contains information about Tech. leads (identified by tid) and Projects (identified by pno). Tech. leads manage projects. You are given some situations concerning the manage relationship set. For each situation, draw an ER diagram for it using the notation taught in our class (assuming no further constrains hold). a. Tech. leads can manage the same project in different calendar years, and each year must be recorded. b. Tech. leads can manage the same project in several calendar years, and only the current year needs to be recorded. c. Every project is managed by exactly one tech. lead. d. Every tech. lead manages exactly one project, and every project must be managed by at least one tech. lead. e. Suppose some projects can be jointly managed by a team of tech. leads. Model this situation by introducing additional entity sets and relationship sets as needed. (Here, we assume that only the latest calendar year of a project nee to be recorded; and a leading team may consist multiple tech. leads, i.e., a tech. lead may be treated as a team of one tech. lead.) one or