Fact 1: Plants release oxygen during the day but not during the night.

Fact 2: Bulbs planted over the years include tulips and daffodils.

Given the two facts above, answer the question "what releases oxygen during the day but not during the night?" with the following options: 
- h2o
 - tulips
 - Oak
 - humans
 - flowers
 - Trees
 - Conifers
 - gravity


 

  tulips       