5. Nuclear fusion has not yet been harnessed as an energy source because .

lasers are too dangerous
of a lack of deuterium
the energy is hard to control
sufficient pressure cannot be generated

Gene therapy  can be described as an important application of biotechnology that is used to introduce normal genes into the cell, which possess defective or missing gene. It is significant in treating genetic disorders ( for instance, it cures cystic fibrosis by introducing normal gene for CFTR protein). This means that DNA with defective gene is modified or replaced with a normal DNA segment ( gene).

But it does not always cure the patients who use it ( for instance,   inserted gene may be recognized as foreign by immune system of the patient, which destroys the inserted gene).

Thus, the correct answers are-

It cures all cases of cystic fibrosis.

It involves the modification of DNA.

It attempts to replace a mutated gene with normal DNA.

It does not always cure patients who use it.