The Hippocratic oath is an ancient Greek oath and the first explicit expression of medical ethics. It is still a simbolic ritual for many novel doctors nowadays. It involves promises made by new doctors, for instance, related to confidentiality with the patients (hence, excluding option A) and to the principle of non-maleficence.
This principle of non-maleficence states that it is preferable not to harm the patient to a greater extent, than to do good to him/her. This is established as a barrier to prevent enthusiastic doctors from using treatments only because of trusting on their knowledge and on hunches, without waiting for enough evidence to support their decisions. This is why they can only treat symptoms.
There are many possible reasons but the two most common were christian missionaries who conducted religious missions all over the world in order to convert people, and the second being direct conquest like what Spaniards did in South American countries.
