locate the rows in a dataframe with some criteria

Hi, netters,

This is probably a rookie question but I couldn't find the answer  
after hours of searching and trying.

Suppose there'a a dataframe M:

x     y
10   A
13   B
8     A
 11   A

I want to locate the rows where x >=10 and y="A". I know how to do  
it to vectors by using
which, but how to do it with the dataframe?

Thank you very much!


Zhihua Li