单选题

如何将当前目录下所有pdf文件内容转换成txt文件内容？

Question

A. for *.pdf to *.txt

B. for f in `ls *.pdf` do pdftotext &ldquo$f&rdquo done

C. do `ls *.pdf` pdftotext `*.txt`

D. pdftotext *.pdf *.txt