Briefly compare the following concepts. You may use an example to explain your point$($s$).$

a$.$ Snowflake schema

b$.$ Galaxy schema

c$.$ Star schema

d$.$ Data cleaning

e$.$ Data transformation

Suppose that a data warehouse consists of the three dimensions time, doctor, and patient, and the two measures count and charge, where charge is the fee that a doctor charges a patient for a visit.

a$.$ Enumerate three classes of schemas that are popularly used for modeling data warehouses.

b$.$ Draw a schema diagram for the above data warehouse using one of the schema classes listed in a$.$