НАДО National Gallery is to north of London. *

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The, the, the
16) KFC is a chain of fast food restaurants. Their fast service is means of attracting customers. *

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The, -
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17) Where is Sahara Desert? *

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a
an
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18) What is capital of Italy? *

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the, the
a, the
a, -
19) On their way home they noticed nasty car accident. yellow car ran straight into white lorry. *

a, an, a
a, the, the
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a, a, a
20) Is Indian Ocean bigger than Atlantic Ocean? *

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the, the
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1) The National Gallery is to the north of London.
The article "the" is used before "National Gallery" because it refers to a specific gallery, the one that we are talking about.
"To" is the correct preposition to indicate the location.
"North" is used to specify the direction in which the National Gallery is located from London.

2) KFC is a chain of fast food restaurants. Their fast service is a means of attracting customers.
"KFC" is a specific chain of restaurants, so the article "the" is not needed.
"A" is used before "chain" because it is referring to any chain and not a specific one.
"Their" is used to refer to the fast food restaurants owned by KFC.
"Fast" is used to describe the service provided by the restaurants.
"Means" is used to indicate that the fast service is a method or way of attracting customers.

3) Where is the Sahara Desert?
"The" is used before "Sahara Desert" because it is a specific desert that we are asking about.

4) What is the capital of Italy?
"The" is used before "capital" because it refers to a specific capital.
"Italy" is a specific country, so the article "the" is not needed.

5) On their way home they noticed a nasty car accident. A yellow car ran straight into a white lorry.
"A" is used before "nasty car accident" because we are referring to any car accident and not a specific one.
"A" is also used before "yellow car" and "white lorry" for the same reason.
"Yellow" and "white" are used to describe the colors of the car and lorry respectively.

6) Is the Indian Ocean bigger than the Atlantic Ocean?
"The" is used before "Indian Ocean" and "Atlantic Ocean" because they are specific oceans that we are comparing.