Create a Sprite Animation in Code

I am trying to construct an Animation clip in C#

 AnimationClip animClip = new AnimationClip();            
 Keyframe[] ks;
 ks = new Keyframe[4];
 ks[0] = new Keyframe(0f, 0f);
 ks[1] = new Keyframe(1f, 1f); 
 ks[2] = new Keyframe(2f, 2f); 
 ks[3] = new Keyframe(3f, 3f); 
 AnimationCurve _curve = new  AnimationCurve(ks);            
 animClip.SetCurve("TestAnimation2", typeof(SpriteRenderer), "Sprite", _curve);
 animClip.wrapMode = WrapMode.Loop;
 game_object.AddComponent ("Animation");
 game_object.animation.AddClip (animClip, "AnimationCurve7");
 game_object.animation.Play("AnimationCurve7");

The above works? however :

How do I add a different sprite to each keyframe?