急求帮忙！C#Task如何在子线程在开子线程，并等待子线程中线程结束
如题：
想要在子线程再开线程，并且要最底层的子线程结束以后才进行其他的 操作，因为工作需要。
我写了个例子，如下：
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    public class TaskTest
    {
        long num;
        public TaskTest(long num)
        {
            this.num = num;
        }

        public void Excute()
        {
            if (this.num > 100000)
            {
                TaskFactory fac = new TaskFactory();

                Task[] pTask = new Task[]{
                fac.StartNew(()=>{
                 long i = 0;
                        while (i < 50000)
                        {
                            i++; System.Threading.Thread.Sleep(1);
                            if (i % 1000 == 0) { Console.WriteLine("当前i已循环到:" + i); }
                        }
                }),
                fac.StartNew(()=>{
                long i = 50001;
                   while (i < this.num)
                   {
                       i++; System.Threading.Thread.Sleep(1);
                       if (i % 1000 == 0) { Console.WriteLine("当前i已循环到:" + i); }
                   }
           &