Player won't stop moving when key is released (160430)

[animeman0]

As it says in the title player won’t stop moving when key is released and I can’t figure out why.
using UnityEngine;
using System.Collections;

public class Controller : MonoBehaviour {

	public float Speed = 5f;
	Rigidbody2D rb;
	// Use this for initialization
	void Start () {
		rb = GetComponent<Rigidbody2D> ();

	}

	void FixedUpdate () {

		if (Input.GetKeyDown (KeyCode.W)) 
		{
			rb.velocity = new Vector2 (rb.velocity.x, Speed);
			if (Input.GetKeyUp (KeyCode.W)) 
			{
				rb.velocity = new Vector2 (0,0);
			}
		}
		if (Input.GetKeyDown (KeyCode.S)) 
		{
			rb.velocity = new Vector2 (rb.velocity.x, Speed * -1);
			if (Input.GetKeyUp (KeyCode.S)) 
			{
				rb.velocity = new Vector2 (0,0);
			}
		}
		if (Input.GetKeyDown (KeyCode.A)) 
		{
			rb.velocity = new Vector2 (Speed * -1,rb.velocity.y);
			if (Input.GetKeyUp (KeyCode.A)) 
			{
				rb.velocity = new Vector2 (0,0);
			}
		}
		if (Input.GetKeyDown (KeyCode.D)) 
		{
			rb.velocity = new Vector2 (Speed, rb.velocity.y);
			if (Input.GetKeyUp (KeyCode.D)) 
			{
				rb.velocity = new Vector2 (0,0);
			}
		}
	}
}

[mahdiii]

you wrote two if into together. It is incorrect
if(){
if(){
}
}
you must write as:
if (Input.GetKeyDown (KeyCode.W))
{
…
}
if (Input.GetKeyUp (KeyCode.W))
{
rb.velocity = new Vector2 (0,0);
}
Also you must do the same work for others.