Question

widespread use of fuel cell vehicles poses a danger because
fuel cells emit a greenhouse gas that endangers the ecosystem.
it is hard to build fuel cell vehicles that travel slowly enough to match speed limits.
hydrogen used to power fuel cells could run out and disappear from earth.
it is difficult to safely transport and store hydrogen gas before it is used.

Explanation:

• Analyze Option A: Fuel cell vehicles (using hydrogen) produce water as a by - product, not a greenhouse gas that endangers the ecosystem, so A is wrong.
• Analyze Option B: There is no inherent difficulty in building fuel cell vehicles to match speed limits, this statement is illogical, so B is wrong.
• Analyze Option C: Hydrogen is the most abundant element in the universe and will not "run out and disappear from Earth", so C is wrong.
• Analyze Option D: Hydrogen gas is highly flammable and has low density, making its safe transportation and storage a significant challenge, which is a real danger in the widespread use of fuel cell vehicles.

Answer:

D. It is difficult to safely transport and store hydrogen gas before it is used.