Final answer:
The speed of a rock thrown upward decreases because it is subject to constant acceleration due to gravity, which slows the rock until its velocity reaches zero at the peak of its trajectory.
Step-by-step explanation:
When a rock is thrown straight upward with an initial velocity, its speed decreases as it goes upward. This is because the rock is subject to gravity, which is an acceleration of approximately 9.8 m/s² (often rounded to 10 m/s² for simplicity), and this acceleration is directed downward, towards the center of the Earth. Therefore, the correct option for your question is B) Its acceleration is constant. While the rock's speed diminishes on the way up due to this constant acceleration, the acceleration itself does not change. At the peak of its trajectory, the rock's velocity will be zero momentarily before it begins to fall back towards the ground, increasing its speed again due to gravity.