What is the time complexity of the following dynamic programming implementation of the longest common subsequence problem where length of one string is "m" and the length of the other string is "n"?

#include<stdio.h>
#include<string.h>
int max_num(int a, int b)
{
      if(a > b)
        return a;
      return b;
}
int lcs(char *str1, char *str2)
{
      int i,j,len1,len2;
      len1 = strlen(str1);
      len2 = strlen(str2);
      int arr[len1 + 1][len2 + 1];
      for(i = 0; i <= len1; i++)
          arr[i][0] = 0;
      for(i = 0; i <= len2; i++)
          arr[0][i] = 0;
      for(i = 1; i <= len1; i++)
      {
            for(j = 1; j <= len2; j++)
            {
                 if(str1[i-1] == str2[j - 1])
                  arr[i][j] = 1 + arr[i - 1][j - 1];
                else
                   arr[i][j] = max_num(arr[i - 1][j], arr[i][j - 1]);
            }
      }
      return arr[len1][len2];
}
int main()
{
      char str1[] = " abcedfg", str2[] = "bcdfh";
      int ans = lcs(str1,str2);
      printf("%d",ans);
      return 0;
}

A. O(n)

B. O(m)

C. O(m + n)

D. O(mn)

Answer: Option D