Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions.

Long ago prehistoric man began to domesticate a number of wild plants and animals for his own use. This not only provided a more abundant food source but also allowed more people to live on a smaller plot of ground. We tend to forget that all of our present-day pets, livestock, and food plants were taken from the wild and developed into the forms we know today.

As centuries passed and human cultures evolved and blossomed, humans began to organise their knowledge of nature into the broad field of natural history. One aspect of early natural history concerned the use of plants for drugs and medicine. The early herbalists sometimes overworked their imaginations in this respect. For example, it was widely believed that a plant or part of a plant that resembles an internal organ would cure ailments of that organ. Thus, an extract made from a heartshaped leaf might be prescribed for a person suffering from heart problems.

Nevertheless, the overall contributions of these early observers provided the rudiments of our present knowledge of drugs and their uses.

The word “This” in the first paragraph refers to ________.