How to search using multiple field USING MVC

Problem

I need to search by ClassID,SectionID,InstructorID DROPDOWNLISTS and i can have options if one drop down have null value or not

search i need is (classID =NULL OR NOT)AND (sectionid =null or not)AND(InstructorID=Null or not)

I only can search by one drop down menu InstructorID BUT SECTIONID,ClassID Dropdown how to do that .

I have Model schedules as following :

This schedule model have foreign key for more tables as Class,Section,Instructor,Calender

All these tables have relations with table schedule

I need to get data from schedule based on ClassID,SectionID,InstructorID DROPDOWNLISTS

But i cannot do search by multiple fields

I only can search by InstructorID FIELD

What I try

Index action inside schedule controller

code below working but only InstructorID Drop down not all drop down

public ActionResult Index()
{
ViewBag.InstructorID = new SelectList(db.Instructors, "InstructorID", "InstructorName");
var schedule = db.schedule.Include(s => s.calender).Include(s => s.classes).Include(s => s.courses).Include(s => s.instructores).Include(s => s.section);
return View(schedule.ToList());
}
[HttpPost]
public ActionResult Index(int InstructorID)
{
ViewBag.InstructorID = new SelectList(db.Instructors, "InstructorID", "InstructorName");
var schedule = db.schedule.Include(s => s.calender).Include(s => s.classes).Include(s => s.courses).Include(s => s.instructores).Include(s => s.section).Where(s=>s.InstructorID==InstructorID);
return View(schedule.ToList());
}

In view of Index action in schedule controller

<p>
@Html.ActionLink("Create New", "Create")
@using (Html.BeginForm())
{
<p>
Title:@Html.DropDownList("InstructorID", null, htmlAttributes: new { @class = "form-control" })<br />
<input type="submit" value="Filter" />
p>
}
p>