Final answer:
To get stronger, all of the options mentioned are important: working harder, adding exercises gradually, and preventing injury.
Step-by-step explanation:
To get stronger, all of the options mentioned are important. Option 1 states that your muscles must work harder, which means increasing the intensity of your exercises. Option 2 suggests slowly adding more exercises to your routine, which helps to challenge your muscles and promote growth. Option 3 emphasizes the importance of gradually increasing the intensity to prevent injury. So, the correct answer is Option 4: All of the above.