Why are autotrophic organisms considered essential to all other life on earth

Ineed to know why d is the correct answer asap

Can create a hboth of these instruments can measure wind speed. doppler radar and psychrometer anemometer and hygrometer doppler radar and anemometer radiosonde and psychrometer

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.