Need help with correct punctuation?

Question

Which sentence is correctly punctuated?

a. Important changes have occurred in the composition

of the American market during the past 15 years.

b. Lou Chao is a creative, conscientious teacher, and has

taught at many schools.

c. The Middle East sheikdom of Bahrain, is a very

wealthy country.

2. what sentence is correctly punctuated?

a. An operating system is the set of instructions, that

directs the overall operations of the computer.

b. While database management systems have existed for

some time, they originally were large, unwieldy, and

difficult to use.

c. The cost and size of computers are decreasing, almost

as rapidly as processing power is increasing.

Answer 1

1. A.

It is a full and short sentence which only needs a full stop, the others have commas in inappropriate places thus breaking up the sentence.

2.  B.

The comma again breaks the sentences of the other options.

Answer 2

1. a

2. b

Answer 3

A

B

Answer 4

1a and 2b