Please submit a very clear ER diagram for the following case.  
Suppose that you want to design and implement a DBMS for a pharmacy. The following requirements that you have gather:
- Patients are identified by an SSN, and their names, addresses, and ages must be ecorded.
- Doctors are identified by an SSN. For each doctor, the name, specialty, and years f experience must be recorded.
- Each pharmaceutical company is identified by name and has a phone number.
- For each drug, the trade name and formula must be recorded. Each drug is sold by a given pharmaceutical company, and the trade name identifies a drug uniquely from among the products of that company. If a pharmaceutical company is deleted, you need not keep track of its products any longer.
-Each pharmacy has a name, address, and phone number.
- Every patient is checked by doctor. Every doctor has at least one patient.
- Each pharmacy sells several drugs and has a price for each. A drug could be sold at several pharmacies, and the price could vary from one pharmacy to another.
- Doctors prescribe drugs for patients. A doctor could prescribe one or more drugs for several patients, and a patient could obtain prescriptions from several doctors.
-Each prescription has a date and a quantity associated with it. You can assume that, if a doctor prescribes the same drug for the same patient more than once, only the last such prescription needs to be stored.