Подберите не определённый артикуль в данных предложениях (если они необходимы) a)

a). The book is big. The particular book, not in general.

b). Lets go for a walk. The fixed phrase.

c). Mary is a student. One of the students.

d). (No article) Oil is (no article) liquid. Well-known fact (true in general, thats why the is not needed). Both nouns are uncountable, that is why a must not be used.

e) (No article) Russia is a big country. Names of countries are used without the definite article. One of the big countries (article a).