Yes according to the Asrock Manual you can put them in A2 and B2
2.5 Installation of Memory Modules (DIMM)
This motherboard provides four 240-pin DDR3 (Double Data Rate 3) DIMM
slots, and supports Dual Channel Memory Technology. For dual channel confi
guration, you always need to install identical (the same brand, speed, size
and chip-type) DDR3 DIMM pair in the slots: You have to install identical
DDR3 DIMMs in Dual Channel A (DDR3_A1 and DDR3_B1; Black slots; see p.2
No. 6) or identical DDR3 DIMMs in Dual Channel B (DDR3_A2 and DDR3_
B2; Black slots; see p.2 No. 7), so that Dual Channel Memory Technology can
be activated. This motherboard also allows you to install four DDR3 DIMMs
for dual channel confi guration, please install identical DDR3 DIMMs in all four
slots.
1. If you want to install two memory modules, for optimal compatibility
and reliability, it is recommended to install them in the slots: DDR3_
A1 and DDR3_B1, or DDR3_A2 and DDR3_B2.
2. If only one memory module or three memory module