Excel 2007 : count the content of cells

Hi all,

assumed i have the following list

Column A - Column B - Column C
client - Delivering/24 - Delivering/hour

a 09:00 (O'clock) 2
a 09:10 (O'clock) 2
b 09:00 (O'clock) 1
b 10:01 (O'clock) 2
b 10:10 (O'clock) 2
c 11:09 (O'clock) 1
c 9:00 (O'clock) 1
a 09:00 (O'clock) 1
a 08:10 (O'clock) 1


how can i build the column C so that for each client i have to calculate how many deliveries via hoer are made and out the result in column C

thanks for supports