A.I. Explanation:

Correct answer: addition

Explanation: The word 'addition' is a noun that fits grammatically as it serves as the subject complement in the sentence, indicating that the Christmas tree is a new element in holiday traditions. Contextually, it aligns with the idea of the Christmas tree being a relatively recent practice.

Meaning: 'Addition' refers to something that has been added to a group or list.

Wrong options:

• review: 'Review' is a noun or verb that refers to examining or assessing something, which does not fit the context of describing a new tradition.
• construct: 'Construct' is a verb that means to build or create, which does not fit as it does not describe the Christmas tree's role in traditions.
• submerged: 'Submerged' is an adjective that means underwater or hidden, which is irrelevant in this context of holiday traditions.

Correct answer: appeared

Explanation: The verb 'appeared' fits grammatically as it is in the past tense, matching the context of discussing when Christianity came into existence. It indicates that Christianity came into view or became known before the Christmas tree tradition.

Meaning: 'Appeared' means to come into sight or become visible.

Wrong options:

• review: 'Review' does not fit as it is unrelated to the concept of existence or emergence in this context.
• construct: 'Construct' is a verb that means to build, which does not relate to the emergence of Christianity.
• submerged: 'Submerged' is an adjective that means hidden or underwater, which does not apply to the context of Christianity's appearance.

Correct answer: decorate

Explanation: The verb 'decorate' fits grammatically as it is in the base form, which is appropriate following 'to' in the infinitive form. Contextually, it describes the action people took with evergreen plants during the winter solstice.

Meaning: 'Decorate' means to make something look more attractive by adding items to it.

Wrong options:

• review: 'Review' does not fit as it does not describe an action related to beautifying or enhancing homes.
• construct: 'Construct' is a verb that means to build, which does not relate to the act of making homes attractive for celebrations.
• submerged: 'Submerged' is an adjective that means hidden or underwater, which is irrelevant in the context of decorating homes.