Cause is what happens and effect is the result of that action
The answer is the second one.
In order for Luke to write his Gospel, he had to go to another sources, since we wasn't himself an apostle (one of the twelve first followers of Jesus Christ)
To tihs day, there is a lot of controversy as of Luke's sources and even his identity. One of the few things we know for sure is he took writings from apostles Mark and Mathew. From the list of responses, we can also pick the name Paul, since Luke is named in the bible "a companion of Paul"
