Write the formula for the polyatomic ion in nh4f

Iknow the answer is a but i don't know why

Which real-world scenarios below represent physical and chemical changes? -running a car -exploding fireworks -mixing water and powdered drink mix -combining oil and vinegar to make salad dressing -taking aspirin for a headache -diluting bleach with water-digesting dinner-spreading peanut butter on bread

Why is retrograde motion an apparent motion