First of all, it's important to note whether the sentence is referring to plural or singular noun. It mentions "los tratamientos", therefore it is plural which eliminates A and B.
Second of all, the phrase "es dudoso que..." is commonly associated with subjunctive. Curar is the original form of the word which in subjunctive changes ar to er tenses.
Therefore your answer is C. curen because it is both subjunctive and plural
Answer:
travel in railroad
get a guide
Explanation:
We can arrive at Machu Pichu in the railroad, several years ago, this mountain area is so hard to build something and unfortunately, people die building the best option to travel to Machu Pichu.
Before the railroad, there are different paths to walk in Machu Pichu, the hardest one, you must contract a guide to do your travel between Inca's path, but there are other paths where just must have a good physical condition.
