There's only a few situations where you don't use an article with singluar nouns, or plural nouns meant in a singular collective sense. Here is an excerpt from a Wikipedia article on articles that gives a good list:
... However, the definite article is not used:
- with generic nouns (plural or uncountable): cars have accelerators, happiness is contagious, referring to cars in general and happiness in
general (compare the happiness I felt yesterday, specifying particular
- with many proper names: John, France, London, etc.
The indefinite article a/an is used only with singular, countable nouns. It indicates
that the referent of the noun phrase is one unspecified member of a
class. For example, the sentence An ugly man was smoking a pipe does
not refer to any specifically known ugly man or pipe.
- No article is used with plural or uncountable nouns when the referent is indefinite (just as in the generic definite case described
above). However, in such situations, the determiner some is often
added (or any in negative contexts and in many questions). For
There are apples in the kitchen or There are some apples in the
kitchen; We do not have information or We do not have any information;
Would you like tea? or Would you like some tea? and* Would you like any
tea?* or Would you like some good tea?
Additionally, articles are not normally used:
- in noun phrases that contain other determiners (my house, this cat, America's history), although one can combine articles with certain
other determiners, as in the many issues, such a child (see English
determiners: Combinations of determiners).
- with pronouns (he, nobody), although again certain combinations are possible (as the one, the many, the few).
preceding noun phrases consisting of a clause or infinitive phrase (what you've done is very good, to surrender is to die).
If it is required to be concise, e.g. in headlines, signs, labels, and notes, articles are often omitted along with certain other
function words. For example, rather than The mayor was attacked, a
newspaper headline might say just Mayor attacked.
Why do you think the speaker B used the article
Generally if the is used for X and you don't know why (e.g. it wasn't mentioned previously), the speaker is talking about a specific X and is expecting you to know which X. X might be something that:
the speaker believes that he/she and you observed directly without talking about it,
the speaker expects you to have read, known, or heard about before,
there is only one of X in the world. E.g. the sky.