Because vs As vs Since

Because, as and since are used to answer the question: ‘Why?’. They join two clauses in the same sentence:

Joe resigned because he wanted to spend more time with his family.

AND because, as and since show the relationship between the ideas in two clauses:

A: Why did you resign from such a well-paid job, Joe?

B: Because I wanted to spend more time with my family.

Because is more common than as and since when the ‘reason’ is the most important thing. The because-clause usually comes after the main clause:

I went to Cyprus for a holiday last October because I knew it would be warm and sunny every day I was there.

As and since are used when the reason is already well-known and/or less important. The as or since-clause often comes at the beginning of the sentence and is separated from the main clause by a comma:

As my family had finished dinner when I got home, I went to this really good burger bar.

( I’m telling you about the burger bar. It’s not so important ‘why’ I went there).

Since it’s your birthday, I’ll make you breakfast in bed (I’m going to make you breakfast.

(I know, and you know, it’s your birthday)

Note! In conversation, so is often used instead of since and as. Theso-clause comes after the main clause.

My family had finished dinner when I got home, so I went to this really good burger bar.

Source: http://www.britishcouncil.org/learnenglish-central-grammar-definitions-conjunctions.htm#because

Ex: Prospecting for gold during the California gold rush was a relatively easy task, because of erosion, prehistoric glacier movement, and ancient, gold-bearing riverbeds thrust to the surface by volcanic activity put gold literally within reach for anybody with a pan or shovel.

A. because of erosion, prehistoric glacier movement, and ancient, gold-bearing riverbeds thrust to the surface by volcanic activity put gold literally within reach for
B. because of erosion, prehistoric glacier movement, and volcanic activity that thrust ancient, gold-bearing riverbeds to the surface, and putting gold literally within reach of
C. owing to erosion, prehistoric glacier movement, and volcanic activity that had thrust ancient, gold-bearing riverbeds to the surface, and putting gold literally within reach of
D. since erosion, prehistoric glacier movement, and volcanic activity that thrust ancient, gold-bearing riverbeds to the surface, putting gold literally within reach for
E. since erosion, prehistoric glacier movement, and ancient, gold-bearing riverbeds thrust to the surface by volcanic activity put gold literally within reach of

Ans E