By Christmas, I … for one year.

Analyze the question The sentence is "By Christmas, I ......... for one year." We need the tense that fits a time marker like "By Christmas" (a future point) combined with a duration ("for one year").

Explain the concept When we talk about an action that began earlier, continues up to a specific point in the future, and has lasted for a period of time, we use the future perfect (continuous) tense:

• Future perfect: shall/will have + past participle
• Future perfect continuous: shall/will have been + verb-ing

The phrases "By + future time" and "for + duration" are strong clues for these tenses.

Solution: test each option

• A. shall have been working: Future perfect continuous. It fits "By Christmas" (a future deadline) and "for one year" (duration). ✔
• B. was working: Past continuous. It cannot go with a future time like "By Christmas." ✘
• C. shall work: Simple future. It does not show an action completed or continuing up to a point, so it doesn't suit "for one year" with "By." ✘
• D. have worked: Present perfect. It does not agree with a future reference point. ✘

Answer The correct option is A. shall have been working.

Full sentence: By Christmas, I shall have been working for one year.

Exam tip When you see "By + future time" (by Christmas, by next year, by 2030), think future perfect: shall/will have .... If "for + duration" is also present, the continuous form will have been + -ing is often the best choice.