Bootstrap modal not appearing properly

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HTML

bootstrap

For delete option, I am using a bootstrap modal but it is not visible properly.
Please help me on this. Here is my code :

<td><a href="#basicModal" data-toggle="modal" >Assign</a></td>

Here is the modal code,

<div class="modal fade" id="basicModal" tabindex="-1" role="dialog" aria-labelledby="basicModal" aria-hidden="true">
      <div class="modal-dialog modal-sm">
        <div class="modal-content">
          <div class="modal-header">
            <button type="button" class="close" data-dismiss="modal" aria-hidden="true">×</button>
            <h4 class="modal-title" id="myModalLabel">Basic Modal</h4>
          </div>
          <div class="modal-body">
            <h3>Modal Body</h3>
            <label class="control-label">Subject:</label>
            <select class="form-control" name="gender" id="gender">
		 <option value="">---------Select Subject--------</option> 
						<option>Maths</option>
						<option>Science</option>
						<option>Social</option>

					</select>
			<label class="control-label">Teacher:</label>
            <select class="form-control" name="gender" id="gender">
						 <option value="">---------Select Subject--------</option> 
						<option>Raghavendra M</option>
						<option>Santosh</option>

					</select>
            
          </div>
          <div class="modal-footer">
            <button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
            <button type="button" class="btn btn-primary">Save changes</button>
          </div>
        </div>
      </div>
    </div>

The problem I am facing is modal window is not appearing in center of screen and also it is appearing within a dialog window . How can I solve this.