Final answer:
The New Guineans were prevented from developing metal tools due to lack of raw materials, fear of large predatory animals, and inability to protect themselves from malaria.
Step-by-step explanation:
The New Guineans were prevented from developing metal tools due to a combination of factors. One reason was the lack of easily accessible surface metal deposits, which made it difficult for them to acquire the raw materials needed for metalworking. Additionally, the fear of large predatory animals and the inability to protect themselves from malaria also hindered their development.