In the following passage, some words have been left out. Select the correct answer for the given blank: “… All of which keeps the bird-lover from the East in an ecstasy of surprises until he has _____________ accustomed to his changed environment.”

Introduction / Context:
This cloze test sentence comes from a descriptive passage about birds of different regions. The author is talking about a bird lover from the East who experiences many surprises in the mountains “until he has ______ accustomed to his changed environment.” The blank requires a verb that completes the present perfect passive-like construction “has become accustomed.” Recognising this fixed expression is essential to choosing the correct option.

Given Data / Assumptions:
- Sentence fragment: “until he has _____________ accustomed to his changed environment.” - Options: became, becomes, to be, become. - The surrounding words “has” and “accustomed” suggest a standard collocation. - We assume standard literary English usage of the phrase “become accustomed to”.

Concept / Approach:
The expression “to become accustomed to” means “to get used to” something. In present perfect tense, it takes the form “has become accustomed.” After the auxiliary “has,” we always use the past participle of the main verb, which for “become” is “become” (the same as its base form). The other options either use simple past, simple present, or an infinitive, which do not fit after “has” in this structure. Therefore, we must choose “become” to form “has become accustomed.”

Step-by-Step Solution:
1. Identify the tense: “has” indicates present perfect tense. 2. Recognise the fixed phrase: “become accustomed to” meaning “get used to”. 3. Examine each option: - “became”: simple past, cannot follow “has.” - “becomes”: simple present, also incompatible with “has” in this position. - “to be”: infinitive form, not used after “has” here. - “become”: correctly serves as past participle, forming “has become accustomed.” 4. Select “become” as the only option that fits both grammar and common collocation.

Verification / Alternative check:
Replace the blank with each option and read the sentence: “until he has became accustomed” and “until he has becomes accustomed” are clearly wrong, and “until he has to be accustomed” sounds unnatural and ungrammatical. “Until he has become accustomed to his changed environment” reads smoothly and matches how people naturally talk about getting used to something over time.

Why Other Options Are Wrong:
- became: Does not work with “has” because present perfect requires “has” plus past participle, not simple past. - becomes: Simple present tense does not combine with “has” in this pattern, and would change the meaning and tense. - to be: Forms an infinitive and does not match the grammar of “has become accustomed.”

Common Pitfalls:
A common error is to choose “became” because learners think that anything after “has” must be a past form, but they forget that the correct form is the past participle, not simple past. For verbs like “become,” the base form, past participle, and sometimes present form can look similar or identical, which can be confusing. Memorising common collocations such as “has become accustomed” helps to avoid such mistakes in cloze passages.

Final Answer:
The correct word for the blank is become, giving the phrase “until he has become accustomed to his changed environment.”