Final answer:
The correct expression representing the bird's flight from an altitude of 100 feet to 120 feet is option D (100 + (-30) + 50), which simplifies to 120 feet, indicating the correct increase of 20 feet.
Step-by-step explanation:
To determine which expression best represents the flight of the bird that started at an altitude of 100 feet and ends at 120 feet, we need to look for an expression that reflects a net increase of 20 feet in altitude. If we analyze option A (100 + 30 - (-50)), we can simplify it to 100 + 30 + 50, which equals 180 feet. This shows an increase of 80 feet, which is not correct. Option B (100 + (-30) + (-50)) equals 100 - 30 - 50, which is 20 feet, indicating a decrease of 80 feet, again incorrect. Option C (100 + 30 + 50) equals 180 feet, which is the same as option A and not the correct increase. Lastly, option D (100 + (-30) + 50) simplifies to 100 - 30 + 50, which equals 120 feet. This correctly shows an increase of 20 feet from the starting altitude, therefore option D is the correct expression that represents the bird's flight.