Read the following passage about HGH and mark the letter A, B, C, or D to indicate the correct option that best fits each of the numbered blanks from 18 to 22.

HGH (Human Growth Hormone) could be the favourite drug at the next athletic world championships, and we might never know it. (18) ___________. Scientists are allowed to make the drug, and it is also legal to take HGH in most countries. Considered a wonder drug for children, (19) ___________.
However, some athletes are now taking the drug. HGH helps add muscle in adults, and recent research by the World Anti-Doping Agency shows that HGH may improve a sprinter’s time by 5%. Some athletes say that HGH definitely makes them stronger. It also helps them to recover more quickly from injuries.
(20) ____________, HGH is almost impossible to test for. The drug is completely natural, and it will only show in tests for around 24 hours after taking it. Testing is usually done only during competitions, but athletes use HGH during training, so it is very difficult to know who has used the drug. This makes it very attractive for some athletes. Scientists are developing a new test (21) ___________. But it isn’t going to be easy. Everybody has different levels of natural HGH in their body.
For this reason, scientists are warning athletes of the possible problems with the drug. Research shows that HGH can give people headaches, pains and - more dangerously - bigger hearts. Finally, high levels of HGH increase the risk of cancer. But is this enough to stop athletes from taking it? Scientists don’t think so. (22) ___________.
(Adapted from Solutions)