Nowadays, biotechnology is causing a new revolution in the field of medicine and health, and people are looking forward to using biotechnology to produce effective therapeutic drugs. Some biologically active substances with specific therapeutic effects, such as enzymes, hormones, vaccines, interferons, immunoglobulins, Interleukins, growth factors, etc. , traditional production processes are mainly extracted from animal organs, blood and urine. Due to limited resources and minimal content, it is difficult and expensive to use a high-purity separation process. Biological genetic engineering methods to produce these drugs can solve these problems well. What is the most accurate understanding of "these difficulties"? ['Limited resources and minimal content.', 'A high purity separation process is difficult and expensive.', 'Limited resources, minimal content, and high-purity separation processes are difficult.', 'Limited resources and minimal content make them expensive.']