thomas edison began serious research into developing a practical incandescent lamp in 1878. edison filed his first patent application for "improvement in electric lights" on october 14, 1878. in the three decades before this numerous similar patents had been filed by other inventors. after many experiments with platinum and other metal filaments, edison returned to a carbon filament. the first successful test was on october 22, 1879, and lasted 13.5 hours. edison continued to improve this design and by november 4, 1879, filed for a u. s. patent for an electric lamp using "a carbon filament or strip coiled and connected to platina contact wires." although the patent described several ways of creating the carbon filament including using "cotton and linen thread, wood splints, papers coiled in various ways," it was not until several months after the patent was granted that edison and his team discovered that a carbonized bamboo filament could last over 1200 hours.

which sentence represents a digression which is not relevant to the point of the passage?
a) the first successful test was on october 22, 1879, and lasted 13.5 hours.
b) thomas edison began serious research into developing a practical incandescent lamp in 1878.
c) in the three decades before this numerous similar patents had been filed by other inventors.
d) after many experiments with platinum and other metal filaments, edison returned to a carbon filament.