Fill in the blanks with 'a' or 'an'.
a. ___ universal problem.
b. _ eucalyptus tree.
c. ___ UNESCO worker.

The correct answer is Option B (a-a, b-a, c-a).

Key Points:
• 'A' is used before singular countable nouns beginning with a consonant sound, even if the letter is a vowel.
• 'universal': begins with consonant sound /juː/ (य) → a universal problem.
• 'eucalyptus': begins with consonant sound /juː/ (य) → a eucalyptus tree.
• 'UNESCO': pronounced as acronym beginning with consonant sound /juː/ (य) → a UNESCO worker.
• Option 2 (a-a, b-a, c-a).
• All three words begin with the consonant semi-vowel sound /j/ ('yoo' sound). Therefore, 'a' is used in all three blanks: a universal problem, a eucalyptus tree, a UNESCO worker.
• Key Takeaway: Words starting with a /juː/ sound (universal, eucalyptus, UNESCO) take the indefinite article 'a'.