This step is a non-restrictive attributive clause guided by where. Decorative drawers and places where famous paintings and jewels are stored must be guided by where.
Tom was one of the first professional tour guides. With his help, Mike climbed the mountain.
You can easily rule out option A. If you choose A, Tom is the third person singular, followed by helps, right? In fact, with whose is Tom's help, and help is a noun here. The key to doing this problem is to find the subject, predicate and object of the sentence correctly. Tom is one of them ... the middle part is unrestricted.
About 1000 species are on the verge of extinction, and the rate of species extinction is accelerating.
The second half of the sentence is already a complete sentence, and it is being used ..., so the choice must be based on the clause that modifies the subject "speed", and the preposition collocation is fixed, so which one is chosen.
The duty of those assistants is to do research in the laboratory. This compound sentence contains stressed sentences. It is/was. There is no single sentence pattern, only tense sentence patterns. The fixed sentence pattern is it is/it was. Modifier and subject part, so choose C.
I hope I can help you. You can experience it carefully. These sentences are all about attributive clauses, including restrictive and non-restrictive, which belong to grammatical content.