13.I saw a place where participants, either singly or in groups, formed complex systems that very much resembledthe workings of modern capitalist societies.
14.According to our experience, abrupt and strongly marked variations occur in our domesticated productions, singly and at rather long intervals of time.
18.Carboxylic acids are a type of organic compound characterized by a carbon (C) atom doubly bonded to an oxygen (O) atom and singly bonded to a hydroxyl group (-OH) .
19.It is first met on the upper margin of the silver fir zone, singly, in what appears as chance situations without making much impression on the general forest.
20.In the court-yard a cock crowed, others near by responded; then from the village, first singly, interrupting each other, then mingling into one chorus, was heard the crowing of all the cocks.