The answer is C: Because the fire trucks arrived immediately, the house was saved.
A causal relationship is characterized by presenting a reason for something else, an effect, happening. It can be expressed in many ways, but it is important to clearly stipulate which statement is the cause and which is the effect. When using the word because, one always refers to a causal relationship. In this case, the fire trucks arriving immediately is the cause for the house having being saved.
Since the buildings during this time period were being built bigger and bigger there was more demand for elevators so people could get from floor to floor. Hope this helped :)
