Hint:

The main verb is 打 dă to open, and the result of the action 打 dă to open is that the two parts of the case will be 开 kāi apart. The correct complement is 开 kāi.

[Answer] [Comments]

Answer:

把书包打开

[Comments]

Comment:

One can also say 打开书包 dă kāi shūbāo.  打 dă is a colloquial usage, and another way to say it is 开开书包 kāi kāi shūbāo. As you may have noticed that both characters 开 are pronounced in the first tone, unlike the duplicated verb 开开 kāikai.

There are more usages of the verb 打dă. 打can also mean to fetch a liquid-like object, e.g., 打水 dă shuǐ to fetch water, 打酱油 dă jiàngyóu to buy some soy sauce. 打听 dă tīng find out information, make an inquiry.

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