The words/phrases in brackets are MY answers; the book answers are included afterward. Please provide an explanation for these errors. Since these sentences are not in context, I just want to know if they are grammatical as is. Sometimes, however, I think the book, Mcgraw-Hill, might be wrong.
19) In fact, Giant Pandas [ are by nature ] extremely solitary animals, usually avoiding direct contact with other animals.
ans: [ ,are by nature, ]
20) In [their dense] habitat, the black and white coat of the Giant Pandas may help make them more conspicuous to each other.
I think this is wrong because "in their dense habitat" modifies "the black and white coat." But I'm not sure because the first phrase is a prepositional phrase so maybe this sentence is genuine.
47) A new subdivision of homes [ building on ] a once thickly wooded plot of land often devastates the natural beauty of the area.
[ built on ], i know built on works, but does building on work as well?
50) Many builders take great care to maintain as much of the natural landscape as possible by keeping mature trees untouched [whenever possible ].
[omit], the reason the book provides is that "whenever possible" is redundant because "as possible" is there. what?