The migrant labourers are as indispensable to their home provinces as they are to the factories, performing a role similar to that played by Filipino workers in Hong Kong who repatriate income earned overseas.
Grammatically, both "B" and "D" are correct, but "B" is correct in meaning, in bringing out the similarity of the roles played by the migrant laborurers and Filipino workers, in sending their earnings back home.
The migrant workers are indispensable to the factories because they provide a source of cheap labour to allow the factories to cut down their production costs. The migrant workers are also indispensable to their home provinces, because they send their earnings back home, which help boost the economy of those provinces.
Therefore the phrase "to the factories" refers to the adjective "indispensible", which describes the subject "the migrant workers". In here "to" is a preposition working with the adjective "indispensible" to link to the object "the factories".
Originally there were two separate sentences:
1. the migrant workers are indispensable to their home provinces.
2. the migrant workers are indispensable to the factories.
These two sentences were then joined together by using the conjunction "as ????? as" to become one sentence:
The migrant workers are as indispensable to their home provinces as they are to the factories.
The structure is different form "I am to do" because "to do" is an infinitive.
Q2. 答案是 C. to adding extra transport services.
The sentence indicates that there is "a range of tools", so it identifies only the first (raising the price of tickets) and the last tool (adding extra transport services) in the range, by using "from" to start the range, and "to" to end the range.
In this paragraph, "where" refers to the place, such as a city or a country, where the policies of using tools to make people take their holidays at different times fail.