C. Three isotopes of the same element because the number of electrons and protons remain the same but the number of neutrons in the nucleus are different. This means that the isotopes have different masses to each other but the same chemical properties.
The average velocity of Sandy is given by the total distance covered S divided by the total time taken t:
The total distance covered is while the total time taken is 2 hours + half an hour (for the rest) + 1 hour and half, so Therefore, the average velocity is