Extract the answer to the question from the following context. Question: Which of the following is not a characteristic of solar balloons: payload-weight, surface area, or market? Context: A solar balloon is a black balloon that is filled with ordinary air. As sunlight shines on the balloon, the air inside is heated and expands causing an upward buoyancy force, much like an artificially heated hot air balloon. Some solar balloons are large enough for human flight, but usage is generally limited to the toy market as the surface-area to payload-weight ratio is relatively high.
Answer:
market