The relation of flowering to the lengths of light and dark periods was investigated by two experiments. In one experiment,the plants were subjected to dark periods of various lengths while the light periods were kept at $4$ hours. In the second experiment,plants were kept in dark periods of various lengths while the light periods were kept at $16$ hours. The number of flowers formed in relation to these periods is shown in the table. These results indicate that flowering:

Dark periods (hours)	Flowers formed (Light: $4$ hrs / $16$ hrs)
$8$	$0$ / $0$
$10$	$0$ / $0$
$12$	$4$ / $6$
$14$	$5$ / $7$
$16$	$5$ / $8$

• A
  Requires long days
• B
  Is initiated by a long light period
• C
  Is due to short light periods
• D
  Requires a minimum dark period