D because they are complete systems
There are many answers to your question but one way is the evaporation cycle.
The correct answer is option B
Sperm cells when pollinated by the insect to the female body part stay there for a while til the part of the flower determines its suitability and once it is done the sperm travels down through the pollen tube.
Cell theory is one of the foundations of biology. The observation of Robert Hooke,Anton Van Leeuewenhoek,Matthias Schleiden,Theodor Schwann,Rudolf Virchow, and others led to the devolepment of the cell theory.
