Two successive measurements give radar echo times of 80 μs80\ \mu\mathrm{s}80 μs and 60 μs60\ \mu\mathrm{s}60 μs. The wave speed is 3.0 × 108 m/s. What is the change in distance between the detector and the reflecting object?
A decrease of 6.0 km6.0\ \mathrm{km}6.0 km
An increase of 6.0 km6.0\ \mathrm{km}6.0 km
An increase of 3.0 km3.0\ \mathrm{km}3.0 km
A decrease of 3.0 km3.0\ \mathrm{km}3.0 km