Final answer:
When sprinting on a flat surface after increasing resistance, the first step is to lean forward slightly to maintain balance and facilitate power generation.
Step-by-step explanation:
When sprinting on a flat surface after increasing resistance to moderately heavy, the first step to take is A) Lean forward slightly. Leaning helps maintain balance and allows for optimal power generation during the initial acceleration phase, where the sprinter exerts maximum power for a short contact time with the ground, boosting their kinetic energy and preventing slowing down. Initially taking shorter strides can also be beneficial to quickly build speed but to answer the student's question, starting with a slight forward lean is a common practice among sprinters.