问为什么我使用LineRenderer只得到两条线，而不是三条线来创建三角形？

EN

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class DrawLines : MonoBehaviour
{
    private LineRenderer lineRenderer;

    // Start is called before the first frame update
    void Start()
    {
        lineRenderer = GetComponent<LineRenderer>();

        Vector3[] positions = new Vector3[3] { new Vector3(0, 0, 0), new Vector3(-1, 1, 0), new Vector3(1, 1, 0) };
        DrawTriangle(positions);
    }

    // Update is called once per frame
    void Update()
    {

    }

    void DrawTriangle(Vector3[] vertexPositions)
    {
        lineRenderer.positionCount = 3;
        lineRenderer.SetPositions(vertexPositions);
    }
}