9.
Your Answer is
Correct Answer is C
Explanation
The meaning of this sentence is "the moving horse provides the patient with stable sensory input, and because the speed of the horse can be controlled, the sensory input can also be controlled", this sentence has three short sentences, and two conjunctions are required. Only Item C is correct.
Item A: then is an adverb and cannot connect sentences.
Item B: Although the meaning makes sense, the grammar is wrong (there is no conjunction between the sentences).