Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Process | Main energy source for process |
| --- | --- |
| lightning | electricity |
| lunar cycle | gravity |
| water cycle | sunlight |
Question:
What is the main energy source for the process of lightning to occur? Select the correct choice to answer the question or fill in the blank.
Choices:
(A) sunlight
(B) gravity
(C) thermal
(D) electricity
Answer:
(D) electricity