From the choices below, choose all that are correct. I. Sound traveling faster through the air in the Sahara desert than in Antarctica.

II. An astronaut hearing the loud explosion of a ship from a mile away in space.

III. Speeds and frequencies of sounds change as they pass through different mediums.

Find v(t), given acceleration a(t)=7j and initial velocity v(0)=k

I'm thinking it's 2 you are asked to explain the earth's magnetic field. which is the best reply? 1. the earth's magnetic south is similar to the north pole of a magnet. 2. the earth's core has a strong magnetic charge similar to the south pole of a magnet. 3. the earth's geomagnetic south is similar to the south pole of a magnet. 4. the earth's magnetic charge is not centered at either pole; it varies based on location.

Explain why electric current cannot exist if a current doesn't have a voltage source.