Because the 'h' in 'hour' is silent, the word starts with a vowel sound. The correct article is therefore 'an'. 'A hour' would be phonologically incorrect. The reference is first mention and indefinite.

In a building, 'the elevator' is identifiable by situational context. Both parties know it refers to the available elevator system. Using 'a' would be less natural and imply one among many without shared reference. Therefore, 'the elevator' is apt.

The phrase begins with the vowel sound in 'iced', so 'an' is correct. The drink is being introduced for the first time, which supports an indefinite article. 'A' would clash with pronunciation. 'The' would imply a previously identified iced tea.

Clock times and certain time expressions like 'noon', 'midnight', and 'dawn' occur without articles. They function as proper temporal points. Adding 'the' would be nonstandard in this context. Thus, no article is correct.

'Only' typically triggers definiteness because it identifies a unique member of the set. The collocation 'the only' is standard. Indefinite articles are ungrammatical in this construction. Therefore, 'the only student' is correct.

The 'of'-phrase specifies which effects are in question, making the noun phrase definite. 'The effects of pollution' refers to a particular set defined by the prepositional complement. Using 'a' or no article would not capture this specificity. Thus, 'the' is required.

Although 'unique' starts with 'u', it begins with a consonant 'yoo' sound. Hence 'a' is the correct article. 'An unique' would be phonetically incorrect. The item is first mentioned, so the indefinite article is appropriate.

Names of diseases typically occur without articles when referenced generally (e.g., 'diabetes', 'cancer'). The sentence discusses the condition as a category. 'The diabetes' would imply a particular instance or subtype in a specific discourse. Therefore, no article is standard.

The word 'email' begins with a vowel sound /iː/, so 'an' is used. It is also a first mention, which favors the indefinite article. 'A' would be phonetically inappropriate. 'The' would presuppose prior mention of that specific email.

The phrase is made definite by the specific location 'library entrance', which uniquely identifies the referent. The listener can pinpoint this entrance in context. Indefinite articles would create ambiguity. Thus, 'the university library entrance' is correct.