Which of the following statements are true?

a. html document will not be displayed if i do not have closing tags
b. without , the web page will not display the contents
c. adding external css is a good practice for various reasons, including code organization
d. external css always has precedence over others

The answer is: c. Adding external CSS is a good practice for various reasons, including code organization.

Explanation:

External CSS improves maintainability in code.  

Internal CSS has precedence over external CSS, it means that internal will overwrite external CSS, but abuse of internal CSS  often makes hard to maintain HTML code on a large website.

Furthermore, it is much easier to modify one CSS(external) file that can impact multiple web pages than go into every HTML page and modify your styles per page. Many sites have hundred or more pages, go through each one wouldn't be productive.

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