Answer:
Communication between scientists increased.
Technological advancements were made.
Step-by-step explanation:
The options:
Metalworkers began using new metals.
Communication between scientists increased.
The American Civil War started.
Technological advancements were made.
The reasons the number of known elements more than double by 1861 would be because technological advancements have been made and communication between scientists increased.
With the advancement in technology, more equipment become available and research became more sophisticated leading to new discoveries. The new discoveries would not have made much difference if they are not adequately communicated to other scientists.