Answer:
The demand for A will decrease
Step-by-step explanation:
If A and B are substitute goods it means either A and B can be used in the place of the other as they satisfy same needs. Two goods are said to be complementary goods if the use of 1 of the goods requires the use of the other. For example if good A was a car, it's complementary good will be petrol.
Since A and B can be substituted for each other, if the price of a Complementary good for A increases, the demand for A will decrease because the law of demand says that higher prices causes decrease in demand. People will rather buy good B whose complement remains the same in price since it is cheaper.
I hope you find my answer useful.