Total time elapsed is =8.2y
The starting event is the astronaut leaving Earth. The finishing event is the astronaut arriving at the star system. The time between these events on Earth is:
Δt=3.9ly/0.9c
Δt=4.3y
For the astronaut, two events occur at the same position and can be measured with just one clock. Hence,
Δτ

Δτ

Δτ=1.8ly
The total elapsed time is:
T elapsed=Δt+3.9
T elapsed=4.3+3.9
T elapsed=8.2y
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Answer:
A) is a cost to a bystander.
Explanation:
A negative externality is defined as the difference between the social cost and an economic agent from the private cost of an action.
A negative externality is a cost to a bystander as negative externality occurs when a transaction between a buyer and seller affects third party with a loss, which has no involvement in the transaction.
Hence, the correct option is A.
The rate of acceleration of the crate would be 1 m/s^2 because the equation for force is F=ma and when you plug in your numbers you get 10=10a so a=1