Help me with a few questions about grammar

A. Everyone help me answer these questions. If possible, please explain your answer.

I. Choose the word or phrase which best completes each sentence

a. Burglar alarms on cars and houses may act as a ..... to the casual thief.

- Prevention

- Deterrent

- Stopper

- Precaution

b. Don't worry if you run out of money at the weekend. You can always get some from the cash .....

- Desk

- Dispenser

- Counter

- Card

II. Identify the incorrect expression among those underlined:

a. Limestone formations growing downward from the roofs of caves that they are stalactites.

b. All matter is composed of molecules or atoms that are in motion in a constant way.

c. The fact that the earth rotates wasn't known until the 1850s.

B. What is the difference between "OBTAIN" and "GET"?

Thank you very much.