Select the sentence that is punctuated correctly. a) my ancient computer a macbook that is ten years old is definitely on its last legs. b) my ancient computer a macbook that is ten years old is definitely on its last legs. c) my ancient computer a macbook that is ten years old is definitely on its last legs. d) my ancient computer, a macbook that is ten years old, is definitely on its last legs.

D. is the answer...It has the correct punctuation... 

Causes of eutrophication:  
 The main causes of eutrophication are natural run-off of nutrients from the soil and the weathering of rocks run-off of inorganic fertiliser (containing nitrates and phosphates) run-off of manure from farms (containing nitrates, phosphates and ammonia) run-off from erosion (following mining, construction work or poor land use) discharge of detergents (containing phosphates) discharge of partially treated or untreated sewage (containing nitrates and phosphates) In most freshwater lakes  the limiting  nutrient is phosphorus, so an input of phosphorus in the form of phosphate ions (PO43-) results in an increase in biological activity.