21) He is going to ..... Netherlands to visit his grandmother.
Explanation: We use definite article 'the' with the names of countries in the plural.
22) When Peter came back home, he saw ....dog at the door.
Explanation: We are not talking about a particular dog. So, we use the indefinite article 'a'.
23) She takes .... dog for a walk.
Explanation: We are not talking about just any dog. It is a pet dog. So, we use the definite article 'the'.
24) ...... Kangaroos are mostly found in Australia.
Explanation: We are talking about kangaroos in general, so an article is not required.
25) The children went to the zoo and saw .... kangaroos.
Explanation: We are not talking about kangaroos in general, these are particulars kangaroos that were seen in the zoo. So we use the definite article 'the'.