Draw data models for the following situations. in eacn case, give each entity:

a$.$ Farmers can own cows or share cows with other farmers.

b$.$ A track and field meet can have many competitors, and a competitor can participate in more than one event.

c$.$ A patient can have many physicians, and a physician can have many patients.

d$.$ A student can attend more than one class, and the same class can have many students.

e$.$ The Marathoner, a monthly magazine, regularly reports the performance of professional marathon runners. It has asked you to design a database to record the details of all major marathons $($e$.$g$.,$ Boston, London, and Paris$).$ Professional marathon runners compete in several races each year. A race may have thousands of competitors, but only about $200$ or so are professional runners, the ones The Marathoner tracks. For each race, the magazine reports a runner's time and finishing position and some personal details such as name, gender, and age.