optimization techniques of website, pls help?

In optimizing a website, we can divide the optimization techniques into the following categories:

Optimize caching - keeping your applications data and logic off the network altogether
Minimize round-trip times - reducing the number of serial request-response cycles
Minimize request overhead - reducing the size of responses and downloads
Minimize payload size - reducing the size of responses and downloads
Optimize browser rendering - improving the browsers layout of a page

The following lists some simple and effective techniques to optimize a large website. For each of them, identify the category it should belong to and then describe how it helps to improve the performance of a website.

(1) Optimize website images
(2) Make JavaScript and CSS external
(3) Put stylesheets at the top (put CSS in the document head)
(4) Minify JavaScript and CSS
(5) Combine several external CSS into One

My answers:
Optimize caching ( I have no idea)
Minimize round- trip times (5)
Minimize request overhead (i have no idea)
Minimize payload size (1)(4)
Optimize browser rendering (3)

I don't know my answers correct or not, pls help