how to lock the z axis rotation

Hi I am somewhat new to Unity and I have a problem, I want to lock all rotation movement for my camera, I was given a script that made it possible for me to lock the x and y axis but not the z please can you help me. Here is the script.

using UnityEngine;

public class Camera_FPS : MonoBehaviour { public Vector2 rotationLimit; //Limit for the vertical rotation of the camera. X is higher limit, Y is lower limit. public float rotationSpeed = 600.0f; //Vertical rotation speed of the camera public bool invertY = true; //Invert vertical rotation public bool headBobEnabled = true; //Enables/disables head bobbing public Vector2 headBobDistance = new Vector2(0.2f, 0.3f); //The distance the camera travels in the y-axis for the headBobbing effect public float headBobSpeed = 5.0f; //Speed at which the player's head bobs

 private Vector3 initialLocalPos; //Starting local y-position of camera
 private float rot; //Stores the rotation value incremented by mouse movement in the y-axis
 private Transform t; //Reference to the transform component of the game object
 
 public void Awake()
 {
     t = transform; //Cache transform component
     initialLocalPos = t.localPosition; //Store initial local position of the camera
 }
 
 public void Update ()
 {
     //Update rotation
     rot += Input.GetAxis("Mouse Y") * rotationSpeed * ((invertY) ? -1 : 1) * Time.deltaTime;
     if(rot < rotationLimit.y) //Check for lower limit
         rot = rotationLimit.y;
     if(rot > rotationLimit.x) //Check for upper limi
         rot = rotationLimit.x;
     t.localRotation = Quaternion.AngleAxis (rot, Vector3.right); //Apply rotation to transform
     
     
     //Head bobbing
     if(headBobEnabled)
     {
         //Calculate percentage of blend between stationary and bobbing camera positions
         float percentage = Mathf.Min(1, Mathf.Abs(Input.GetAxis("Vertical")) + Mathf.Abs(Input.GetAxis("Horizontal")));
         
         //Calculate desired x position
         float desiredPosX = initialLocalPos.x + headBobDistance.x * Mathf.Sin(Time.time * headBobSpeed + Mathf.PI/2);
         //Blend between stationary and desired x position
         float newX = (initialLocalPos.x * (1 - percentage)) + (desiredPosX * (percentage));
         
         //Calculate desired y position
         float desiredPosY = initialLocalPos.y + headBobDistance.y * Mathf.Sin(Time.time * 2 * headBobSpeed);
         //Blend between stationary and desired y position
         float newY = (initialLocalPos.y * (1 - percentage)) + (desiredPosY * (percentage));

         t.localPosition = new Vector3( newX, newY, t.localPosition.z);
     }
 }

}