Identify the subject in the following sentence: Alex is writing a book.
Alex is writing a book.
The subject in the sentence "Alex is writing a book" is "Alex".
Complete the following task based on the instruction and optional input provided.### Instruction:
Identify the subject in the following sentence: Alex is writing a book.

### Input:
Alex is writing a book.

### Output:
The subject in the sentence "Alex is writing a book" is "Alex".