Which best describes the difference between a seismograph and seismogram?   a. a seismograph is the written record of earthquake activity, while a seismogram is the instrument that detects earthquakes.   b. a seismograph detects earthquake activity, while a seismogram measures the depths of the earth's layers.   c. a seismograph detects earthquake activity, while a seismogram is the written record of that activity.   d. a seismograph detects continental drift, while a seismogram detects plate tectonic movements.