Personal computers first made their appearance in the home in the 1970s, but surely few people would have been able to imagine then that the home computer could evolve into the super-fast, super-sleek machines of today. Nor could they have even thought that handheld mobile gadgets would [use constantly] by all of us, in our desperation to keep up with everything from office correspondence to world news.
The answer key to this exercise mentions only 2 correct answers–'would be being used constantly' or 'would constantly be being used' both of which sound really awkward to me and like something I'd never say. I'd prefer
Nor could they have even thought that handheld mobile gadgets would be used constantly by all of us
What is wrong with this answer? I'm sorry but to me, this sounds like the most elegant and the only possible answer yet the answer key disagrees. Any thoughts on why?