检查字符串是否包含一个单词

Random rnd = new Random(DateTime.Now.Millisecond);

string rt = "tst xtstx xtste tst tst!!";

if (rnd.Next(3) == 0)
{
    string[] replaceWords = { "something", "nice" };
    rt.Replace("tst", replaceWords[rnd.Next(replaceWords.Length - 1)]);
}
if (rnd.Next(3) == 0)
{
    string[] replaceWords = { "cool", "crazy" };
    rt.Replace("xtste", replaceWords[rnd.Next(replaceWords.Length - 1)]);
}
Random rnd = new Random(DateTime.Now.Millisecon