How to change the list items of of dropdown list based on selected item of another list item and database

See more:

C#

ASP.NET

SQL-Server

Dear Programmers,
I am making a student registration form for a computer institute. In which I have two dropdown lists, one is for lecturetimings and another is for availablecourses. I made two tables in database 1 table will save all the information of student when he clicks on submit, another is courses table.
The structure of courses table is:
lectTime ----------SoftwareLab-----------HardwareLab
MondayAM-------MSoffice---------------NULL
TuesdayAM-------C-Language------------BasicHardware
WednesdayAM----MSoffice---------------CCNA
MondayPM-------VisualBasic------------Null
ThursdayPM-----Null-------------------Basic Hardware

In my first dropdown list(i.e "lecturetimings") I have all my lecture timings(MondayAM, TuesdayAM,WednesdayAM, MondayPM and ThursdayPM). Now my requirement is if a user selects any lecturetimings-for Ex:MondayAM, then in another dropdown list it should available course(MS-office), if selectes TuesdayAM then it should show two values(C-Language, BasicHardware).

And another requirement is in my register table it should not allow to insert more than 20 records for each course of each day.(For example MondayAM-MSoffice only 20 students are allowed, TuesdayAM-C-Language 20 students and BasiHardware 20 students allowed).

Anyone kindly solve this with the code.

Thanks in advance