Need answer asap ! which of these elements has four valence electrons? (a). hafnium (hf) (b.) radon (rn) (c.) silicon (si) (d.) sulfur (s)

What are segments of dna that code for a particular trait. a combination of dna and proteins make up what

You are driving directly behind a pickup truck, going at the same speed as the truck. a crate falls from the bed of the truck to the road. (a) will your car hit the crate before the crate hits the road if you neither brake nor swerve? (b) during the fall, is the horizontal speed of the crate more than, less than, or the same as that of the truck?

Ahot-air balloon takes off from the ground traveling vertically with a constant upward acceleration of magnitude g/4. after time interval δt, a crew member releases a ballast sandbag from the basket attached to the balloon. how many seconds does it take the sandbag to reach the ground? (express your answer in terms of δt)