How come the Indians were at a disadvantage against the Spanish conquistadors?

The Indians, better known as Native Americans, had two main disadvantages against the Spanish:

The Native Americans did not have as much technology. In fact, many tribes were still in the Stone Age, while the Spanish already had firerarms. They also lacked horses, an important asset in military matters that the Spanish possessed.The Native Americans did not have antibodies against Eurasian diseases like measles or smallpox. These diseases were brought by th Spanish, and caused massive deads among the Native American populations (up to 90%).

The answer to your question would be b