Step-by-step explanation:
b. scientists can modify viral DNA
d. viruses can direct cells to make specific proteins
The primary reason why viruses are used to deliver therapeutic genes to target cells is that they have the natural ability to enter cells and deliver genetic material. However, this ability can also cause harm to the host organism. Therefore, scientists modify the viral DNA so that it can safely deliver therapeutic genes without causing harm. Additionally, viruses can be engineered to direct cells to make specific proteins, which can be beneficial for treating various diseases. Therefore, options (b) and (d) are correct. Option (a) is incorrect because although viruses can enter cells, they cannot be directly injected into cells. Option (c) is also incorrect because viruses do not carry many beneficial genes; rather, they can be engineered to carry specific therapeutic genes.