Hi vkhu, I'm not a teacher, but considering no one has replied your thread so far, I'll give it a try.
None of the underlined words seem wrong to me too, but If I had to pick one I'd say it's is thought to, maybe it could be plural if you consider the species. So are thought to...
But 'huge numbers' doesn't sound ok to me. I'd say the huge number, or the huge amount.
Student or Learner