In the first blank, article “the” will be used because ‘most’ is a superlative adjective and article “the” is used with superlative adjectives.(concept)
Similarly, in the second blank, ‘greatest’ is superlative adjective, therefore, article “the” will be used.
In the third blank, article “the” will be used because article “the” is used when there is something only one (the moon, the sun, ‘the world’).
In the fourth blank, English word is representing a specific group of people. Therefore, article “the” will be used.

The definite article ‘the’ is used before specific things/places. The sentence is referring to the specific week after the next.

Definite article is used with positions, directions, numbers etc. Hence, ‘underground’ will follow ‘the’ article.

A’ is used before singular nouns used in a general sense. Here, ‘Piano’ is used in a general sense. However, ‘corner’ and ‘room’ are specifically the ones where the piano was kept. Therefore, the correct option is: a; the; the.

In the FIRST blank, article “the” will be used because “Potomac River” is a definite place.
Similarly, article “the” will be used in the SECOND blank because ‘District of Columbia’ is also a “definite place”.

Option C is correct as ‘The’ is used with words with superlatives. ‘Most’ is superlative.

‘The’ is a definite article and is used before specific things/places. The sentence refers to the specific book that was lent.

Here, ‘accident’ is a general class and therefore indefinite article will be used. Word after the blank starts with vowel sound. ‘An’ article will be used.

No article is used before uncountable and abstract nouns or plural countable nouns used in a general sense. Here, ‘concerts’ is a plural countable noun used in a general sense.