How can I define the subject ? During the eighteenth century, communication within and between cities...... at first. a)were difficult b)they were difficult c)difficult d)was difficult Why "were difficult" is not correct here? The diffrence between the living conditions in the country side and in towns...... a)have been eliminated b)have eliminated c)has been eliminated d)has eliminated Why "the living conditions" is not a subject so that the "a" is not correct A reorganization of the brain cells..... during adolescence. a)occurs b)occur c)occuring d)are occurring Why "are occuring" is not true? What is the subject?