Why Are Next Generation DNA Sequencing Technologies Known as Sequencing-

Why are next generation DNA sequencing technologies known as sequencing- by- synthesis?

A) Numerous synthesized fragments of DNA are sequenced to determine which nucleotides were incorporated.
B) Incorporated nucleotides are determined while they are being added to a growing DNA strand.
C) RNA molecules are synthesized off of the DNA templates and the incorporated nucleotides are then determined.
D) The complete DNA strands are synthesized then sequenced.
E) The sequencing occurs during S phase of the cell cycle.