c#中判断变量string a是否为空字符串时（a不为null），下列哪个语法执行最快？

A. a==""

B. string.IsNullOrEmpty(a)

C. a.Length==0

D. a.Equals("")

using System;
using System.Diagnostics;

namespace ConsoleApp2
{
    class Program
    {
        static void CompareExecutedTime()
        {
            const int total = 1000000000;
            var str = string.Empty;
            var stopwatch = new Stopwatch();
            
            stopwatch.Start();
            for (var i = 0; i < total; i++)
            {
                if (str == ""){}
            }
            stopwatch.Stop();
            Console.WriteLine($@"str == """" 的耗时: {stopwatch.Elapsed}");
            stopwatch.Reset();
            
            stopwatch.Start();
            for (var i = 0; i < total; i++)
            {
                if (str == string.Empty){}
            }
            stopwatch.Stop();
            Console.WriteLine($"str == string.Empty 的耗时: {stopwatch.Elapsed}");
            stopwatch.Reset();
            
            stopwatch.Start();
            for (var i = 0; i < total; i++)
            {
                if (string.IsNullOrEmpty(str)){}
            }
            stopwatch.Stop();
            Console.WriteLine($@"string.IsNullOrEmpty(str) 的耗时: {stopwatch.Elapsed}");
            stopwatch.Reset();
            
            stopwatch.Start();
            for (var i = 0; i < total; i++)
            {
                if (str.Length == 0){}
            }
            stopwatch.Stop();
            Console.WriteLine($"str.length == 0 的耗时: {stopwatch.Elapsed}:");
            stopwatch.Reset();
            
            stopwatch.Start();
            for (var i = 0; i < total; i++)
            {
                if (str.Equals("")){}
            }
            stopwatch.Stop();
            Console.WriteLine($@"str.Equals("""") 的耗时: {stopwatch.Elapsed}");
            stopwatch.Reset();
            
            stopwatch.Start();
            for (var i = 0; i < total; i++)
            {
                if (str.Equals(string.Empty)){}
            }
            stopwatch.Stop();
            Console.WriteLine($@"str.Equals(string.Empty) 的耗时: {stopwatch.Elapsed}");
            stopwatch.Reset();
        }
        
        static void Main(string[] args)
        {
            CompareExecutedTime();
        }
    }
}