After obtaining the upgrades through-out the game, go back to the first few levels and you can get a better grade with the upgrades

After you defeat the third boss, you can go back and find these rare items to unlock special abilities:

Item ---------------------- Location
Fixed Radar Unit ---------- Located in Level 3
Repair Unit --------------- Located in Level 6
Sensible Shoes ------------ Located in Level 8
Camouflage Unit ----------- Located in Level 11
Auto Repair System -------- Located in Level 12
Item Boost Unit ----------- Located in Level 18
Capture Range Expansion --- Located in Level 20