The following graph shows how the stock price of a company has fluctuated over the three years.

Another company was looking to invest their earnings in buying stock of this company. When would have been a good time to purchase stock, and when would have been a good time to sell to maximize their profit?
 

1-Jan-07 1-Feb-07 1-Mar-07- 1-Apr-07 1-May-07 1-Jun-07 1-Jul-07 1-Aug-07 1-Sep-07 1-Oct-07 1-Nov-07 1-Dec-07 1-Jan-08 1-Feb-08 1-Mar-08 1-Apr-08 1-May-08. 1-Jun-08 1-Jul-08 1-Aug-08 1-Sep-08 1-Oct-08 1-Nov-081 1-Dec-08 1-Jan-09- 1-Feb-09 1-Mar-09 1-Apr-09 1-May-09 1-Jun-09 1-Jul-09 1-Aug-09 1-Sep-09 1-Oct-091 1-Nov-09 1-Dec-09 1-Jan-10 1-Feb-10 1-Mar-10. 1-Apr-10 UT 8 B Stock Price 2007 to 2010