How to Add Force on the X and Z but not Y

Unity Engine
1

I am making a little rolling ball game, and I have a good movement system, but there is one problem with it. The way it works is pretty much, which ever way I am looking, I can use the WASD keys to move relative to that direction. But, it also works for up and down. I only want the movement force to be applied to the X and Z axis. How do I get it to NOT work for the Y axis. To be clear, I still want to be able to move on the Y axis, I just don’t want THIS movement system to move me on the Y. I am still VERY new to programming, so any help would be appreciated. Here is my movement script.

using UnityEngine;
using System.Collections;

public class PlayerMovement : MonoBehaviour {

    public float speed;
    public GameObject center;

    private Rigidbody centerRig;

    void Start()
    {
        centerRig = GetComponent<Rigidbody>();
    }

    void Update () {

        if (Input.GetKey(KeyCode.W))
        {
            centerRig.AddForce(center.transform.forward * speed);
        }

        if (Input.GetKey(KeyCode.S))
        {
            centerRig.AddForce(center.transform.forward * -speed);
        }

        if (Input.GetKey(KeyCode.D))
        {
            centerRig.AddForce(center.transform.right * speed);
        }

        if (Input.GetKey(KeyCode.A))
        {
            centerRig.AddForce(center.transform.right * -speed);
        }
    }
}
2

Is the object ever going to move in the y? You can simply freeze the position.

Otherwise you can use Vector3.forward instead of transform.forward.

3

Can you try to explain that better. What exactly you don’t want to happen

4

It will be moving on the Y a lot. Up and down ramps and possibly jumping. And I already tried the Vector3.forward, but that only moves it in one direction (on the X I believe). I need it to move forward from any perspective that I’m facing. I have it all working right, except I need it to not add force on the Y axis.