First, I want to let you know, there is Single beam and Dual Beam Bulb(BI-Xenon) types.
a) If your car use one bulb to give High beam and another bulb to give Low beam, that means your current bulb is Single beam model, so it should be one of these types: H1/H3/H4(HB2)/H7/H11/9004/9005(HB3)/9006(HB4)/9007/880/881. 
You should buy the HID KIT accordding to your current bulb types. While, one KIT can give you only High Or Low, if you want both, you have to buy 2 sets.
b) If your car use one bulb to give both High and Low beams, that means your current bulb have two filaments, or we called it Dual Beam Bulb, so you should buy a BI-Xenon HID Kit, like H4-3(HB2),9004-3,9007-3,H13-3. You should buy the Bi-Xenon HID KIT accordding to your current bulb types. Bi-Xenon bulbs are telescopic model, it can move so you can have one bulb to give you High and Low. One set of BI-Xenon HID KIT can give you both High and Low with HID.
c) There is another bulb type, H4-2(HB2), this kind of bulb is similar to H4-3, it can give you both High and Low beam, but one of the beam is halogen, another is HID. If your car use H4 type bulb to give both High and Low, you can buy H4-2 or H4-3 HID KIT. 

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