for each loops ; String cannot be converted to int?

I need to acess 2D array and write some information there. I want to use for-each loop for this. Here is my code. But when executing it says ;

q13.java:24: error: incompatible types: String[] cannot be converted to int
                                                wi.write(St[a][b] + "\t");
                                                            ^
q13.java:24: error: incompatible types: String[] cannot be converted to int
                                                wi.write(St[a][b] + "\t");

What's wrong in my code?

import java.io.*;
class q13{
    public static void main(String[] args) 
    {
        String St[][]={ {"STUDENT NO", "STII", "SPD", "PS", "DCCNII", "SEI"}, 
                {"DIT/10/C1/0001", "A", "A-", "B+", "A", "A"},
                {"DIT/10/M2/0123", "C-" ,"C" ,"B" ,"B", "B+"},
                {"DIT/10/M1/0054", "D" ,"C-" ,"C" ,"B-", "B"},
                {"DIT/10/M1/0025", "A" ,"A" ,"A-", "A", "A"},
                {"DIT/10/C2/1254", "C" ,"C-" ,"B" ,"B+", "B"}  };

        try{
            BufferedWriter wi = new BufferedWriter(new FileWriter(".\\st_info.txt"));

                for(String[] a:St)
                {
                    for(String[] b:St)
                    {
                        wi.write(St[a][b] + "\t");
                    }

                   wi.write("\r\n");
                   wi.newLine();
                } 

            wi.close();
        }
        catch(IOException ex)
        {
        System.out.print(ex);
        }

    }

}