Urgent answers neede???

1) In select-options,how to get the default values as current month of first dte and last date by default? Eg: 1/12/2008 and 31/12/2008??

2)In selection screen i have three fields.. Plant,,mat no., and material group.. If i input plant how do i get the mat no and material group based on plant dynamically??

3)what is table type? and what is the difference between table type and structure?