The element of symbols: symbols to know

Chemical symbols are shorten abbreviations of the names of the 109 known elements. The use of symbols for the chemical elements existed long before a systematic method was developed. the alchemist associated the symbols of the planets not only with the days of the week, but also with the seven metals known at the time: gold, silver, iron, mercury, tin, copper , and lead. By the beginning of the nineteenth century, there were about  26 known elements, but by the beginning of the twentieth century, therer were more than 81.

The correct answer is  b)