Strange question mark characters in post

What would cause the appearance of these strange question mark characters in a Wordpress post? They look like this …

when they should be apostrophes.

When I do a “view source” on my browser, I see the same characters.

The page header is like …

<!DOCTYPE html>
<html dir="ltr" lang="en-US">
<meta charset="UTF-8" />

which is fairly standard for Wordpress.

the 2 floor have a right opinion on this, i have the same problem before, and now is ok.

This is a very common problem. Everything - everything - must have a consistant character encoding. eg.
Files must be saved as UTF-8 (without BOM signature)
The database must use UTF-8 charset and preferably collate as well
The HTTP headers sent with the file from the server must be UTF-8
and the meta tag can specify UTF-8

Of these, the meta tag is the least important IMHO

If you are editing files, check your text editors’s settings
If these are saved and read from a WordPress database, check your wp-config file DEFINEs. *you may need to redo affected posts.