text: optimize next C# method: public static string MakeComplement(string dna)
      {
          StringBuilder sb = new StringBuilder();
          for(int i = 0; i < dna.Length; i++)
          {
  
              switch (dna[i])
              {
                  case 'A':
                      sb.Append('T');
                      break;
                  case 'T':
                      sb.Append('A');
                      break;
                  case 'C':
                      sb.Append('G');
                      break;
                  case 'G':
                      sb.Append('C');
                      break;
                  default:
                      break;
              }
          }
          return sb.ToString();
      }