How do I allocate a character array of length 20 for each element of the array pointed to by last_names?
#include <stdlib.h>
#include <stdio.h>
int main() {
    char **last_names;
    // Assign a dynamically allocated char * array of length 4 to last_names. 
    // Then, allocate a character array of length 20 for each element of the array 
    // pointed to by last_names.
    last_names = (char **) malloc(sizeof(char*)*4);
    for (int i=0; i <= sizeof(last_names); ++i)
    {
        last_names[i] = (char *) malloc(20 * sizeof(char));
    }
    return 0;
}