How to save questoes in an array of pointers

Question

Good night

I come here to try to dispel a programming problem that occurs to me. In this solution, I have a file with several questions, and each question can be a matter of one of the 3 following themes:

1. GEO - Geography
2. MAT - Mathematics
3. HIS - History

If the user in the execution of the program chooses to answer geography questions, then a method that selects only geography questions will be invoked. This method has among other code the following:

while (fgets(pergunta, 150, f) != NULL) // ler ficheiro linha a linha
    {
        sscanf(pergunta,"%ld", &elapsed_seconds); // ler cada poergunta do ficheiro
        length = sizeof(pergunta) - 4; // pegar no tamanho dos primeiros 4 caracteres

        for (int j = 0; j < length; j++)
        {
            if (pergunta[j] == 'G' && pergunta[j + 1] == 'E' && pergunta[j + 2] == 'O') // se a pergunta é de GEO
            {
                for (int i = 0; i <= sizeof(questoes); i++)
                {

                    //Problema 
                    if(!questoes[i]) // se encontrar posicao vazia
                        questoes[i] = pergunta; // guarda pergunta no apontador

                    if (i == sizeof(questoes))
                    {
                        resultadoRetomado = 1;
                        break;
                    }
                }
            }
            else if (pergunta[j] != 'G' && pergunta[j + 1] != 'E' && pergunta[j + 2] != 'O')
            {
                pergunta[j] = '\0';
            }
        }

        }

Problem:If a geography question is found, the pointer char* questoes[50], passed as parameter to the method, it will be covered and the first question found will be stored. I intend and tried to change this algorithm to check if there is any empty position in the pointer set.

Objective: I intend to keep in char* questions[50] a set of 50 questions. How can I change this code to go through the file and save the 50 questions?

From now on Thank you,