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A. In my previous job, I made contributions to my employer such as improving the process efficiency, providing quality service to customers, and increasing cost savings.

B. In my previous job, I made contributions to my employer such as having improved the process efficiency, having provided quality service to customers, and having increased cost savings.

1. Which sentence above is correct in the given context?
2. What is the difference in meaning between using the -ing form and having+past participle?
3. In addition, should it be "contributions I made for my employer" or "contributions I made to my employer"? Thank you.
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1. Which sentence above is correct in the given context?- A in most every context; B is far too awkward to be used.

2. What is the difference in meaning between using the -ing form and having+past participle?- Here, the simple -ing form is timeless; it is a nonfinite verb. Meanwhile, the perfect nonfinite casts the action (here, to the applicant's detriment) further into the past.

3. In addition, should it be "contributions I made for my employer" or "contributions I made to my employer"? -- Contributions I made to my employer's operations/business/success.
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Thank you for helpful response, Mister Micawber.

These contributions actually happened a year ago. In this case, should I use sentence B for an interview? Or A is much preferable since the form is timeless as you've explained?
We know it's a while ago: you have already said 'previous job' and 'made'. Use A: it is the natural approach and is good style. The perfect nonfinite would only be contrived by non-native speakers and nitpickers.
Okay, thank you for your answer and caution given. I would use A then.
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