Final answer:
A strength-training activity focuses on building and toning muscles, such as sit-ups which target the core and upper body muscles.
Step-by-step explanation:
A strength-training activity is an exercise that focuses on building and toning muscles. Sit-ups and push-ups are examples of strength-training exercises that target the core and upper body muscles. Performing sit-ups involves contracting and flexing the abdominal muscles, while also engaging other muscle groups such as the hips and legs.
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