I'll never forget . . . . . . . . first time I saw . . . . . . . . real American Christmas tree.

A. a, the

B. an, no article

C. no article, a

D. the, a

Answer: Option D

Solution(By Examveda Team)

The first blank refers to time, the second to tree.
The time is specified to be the first, definite article of "the" will be used in the first blank.
The tree is preceded by an adjective of real, thus a countable noun "a" will be used in the second blank.