An attempt on the part of the author of this passage to give a more informal tone to his writing occurs in ______.

A．when it was double the number that could live in the enclosure without stress
B．by removing young rats
C．so that there was a constant number of adult rats in the enclosure
D．All of the above are correct

A．Eskimo words are full of forms
B．Eskimo words are difficult to spell
C．Eskimo words are numerous
D．Eskimo words are hard to understand