The model below represents the apparent path of the sun across the sky on march 21 as seen by an observer on earth.
at which latitude is the observer located?
(1) 90° n
(2) 42° n
(3) 23.5° n
(4) 0°

By looking at the model/graph you posted, when the path of the sun is across the sky on March 21, the observer that is located on earth is located at the latitude 0. The correct answer choice will be number 4. 

When a lake, river or other aquatic  system  becomes  eutrophic, it can have serious negative effects on other organisms like fish, birds and even people.if an algal bloom is so bad that it causes wide-spread death  in  the  water, the organisms that die will all sink to the bottom and start to decompose.