1.
a) C5H9N3
b) 
c) N-H (can form hydrogen bonds with water)
2.
a) C16H21N3
b) The similarities are: a two-carbon chain with a nitrogen at the end and a three-atom sequence of N-C-N, the spatial placement of these pieces, and the flat 5-ring of histamine can also be considered similar to the flat 6-ring of the antihistamine.
3. Drugs that produce a physiological response in the body: b, d
Drugs that inhibit the growth of substances that cause infections: a, c, e, f
4.
a) Since becoming widely available, problems have been found in certain dietary supplements. The causes of this include how herbs are sometimes contaminated with germs, pesticides, or heavy metals that are toxic, or are accidentally contaminated by the growth of another plant within the herb’s population which could lead to cases of mistaken identity in similar looking plants. Other causes include how the labels on these supplements do not add up to what is actually in them; supplements might contain more or fewer ingredients as listen on the label, or the amount might be different from that stated on the label. Furthermore, some herbal suppliers (those who grow, harvest, or sell the crops) may mix or even substitute their crops with less expensive or more readily available plants for profit purposes. Given the global market, all of these problems can make it harder for a company to be sure that what they thought they were buying to make supplements is actually the herb they wanted.
b) All Chinese Proprietary Medicines (CPM) i.e. products in the finished dosage forms (e.g. tablet, capsule, liquid) are regulated by the Health Sciences Authority (HSA) and must comply with a set of safety and quality criteria before they are allowed to be sold in Singapore. In addition, CPM dealers (importers, wholesale dealers and manufacturers) are also required to be licensed by HSA.