What who does high gear mean?

[Thameem]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #519 "English Slang Idioms (103)", question 4

"Richard, it's Max. Sorry I haven't called lately. I have been working overtime -- nights and weekends -- on our new computer system and it's software. We are really in high ......... at the moment and I haven't had much time for anything. I'll call as soon as I get a break."

(a) spirits
(b) volume
(c) gear
(d) solubility

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #519 "English Slang Idioms (103)", answer 4

"Richard, it's Max. Sorry I haven't called lately. I have been working overtime -- nights and weekends -- on our new computer system and it's software. We are really in high gear at the moment and I haven't had much time for anything. I'll call as soon as I get a break."

Correct answer: (c) gear

Your answer was: incorrect
"Richard, it's Max. Sorry I haven't called lately. I have been working overtime -- nights and weekends -- on our new computer system and it's software. We are really in high volume at the moment and I haven't had much time for anything. I'll call as soon as I get a break."
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who does high gear mean? is that mean the shop is running with high sales?

[Thameem]

I think this site is for learner not for some one to advertise their product. Please be aware of that. Alan co-founder of this site must react for this kind of activities.