In dunker's candle experiment, subjects weren't able to find a solution to the candle problem because they could not see past the conventional use of the box. which obstacle to problem solving did the subjects experience?

Functional fixation                              

Explanation:

Functional fixation: In psychology, the term functional fixation refers to the cognitive function or cognitive bias in which a person becomes fixated on a particular function related to an object that prevents him or her from utilizing that specific object in any other way for problem-solving because the person feels that the object works only in a traditional way.

Example: A person uses an object in only a specific way and finds difficulty in using it in an alternative way.

In the question above, the subject experiences functional fixation.                            

  b.

there are isolated pockets of farming, generally around rivers and oases, which are used for irrigation.