it would be d. very simple
the formula for the most stable form of oxygen is o₂, so the subscript is 2.
however, there is a less stable form of oxygen called ozone. its formula is o₃, so the subscript for ozone is 3.
most events like the rising and setting of the sun were used a natural measurement of time until recently.
solar time, which is based on the motion of the sun, is not the only way of measuring time, however. one might keep track of the regular appearance of the full moon. that event occurs once about every 29.5 solar days. the time between appearances of new moons, then, could be used to define a month.
one also can use the position of the stars for measuring time. the system is the same as that used for the sun, since the sun itself is a star. all other stars also rise and set on a regular basis.
although any one of these systems is a satisfactory method for measuring some unit of time, such as a day or a month, the systems may conflict with each other. it is not possible, for example, to fit 365 solar days into 12 or 13 lunar months exactly. this problem creates the need for leap years
i abbreviated most of it but there is a ton more at this link if you still need more.
. c) which measurement tools are used.