Which statement about a signal transduction pathway is true?
a. a signal transduction pathway provides a means for a signal molecule to pass through the cell membrane and initiate a chain of events inside the cell.
b. a signal transduction pathway is nonspecific, which allows the cell to use the same pathway for many different environmental signals.
c. a signal transduction pathway allows a cell to change its external environment to make it more suitable for its needs.
d. a signal transduction pathway allows a single signal molecule to initiate a chain of events that amplify the signal at many steps inside the cell.