Which has a mismatch between subject and modifier?
as i was ducking under an awning, the heavy rain caught me without a coat or an umbrella.
ducking under an awning, the heavy rain caught me without a coat or an umbrella.
ducking under an awning, i was caught without a coat or an umbrella in the heavy rain.

in my opinion, ap calculus was the most difficult. i often struggled with it a lot. however, i have gotten a lot better.

the direction of the magnetic field produced by an electric current will change when the direction of the electric current changes. for example : if a current is flowing in a straight length of wire, the magnetic field is described by a concentric circle (using right-hand rule).