Write the Word or Phrase That Best Completes Each Statement $$\text { With the function } f ( x ) = \log _ { a } x \text {, why can't } x \text { be less than } 0 \text { ? }$$

Write the word or phrase that best completes each statement or answers the question.
- $$\text { With the function } f ( x ) = \log _ { a } x \text {, why can't } x \text { be less than } 0 \text { ? }$$