Strictly speaking, in order for matter of energy to be conserved, a system must a. open b. closed c. unique d. stationary

Closed system.  If the system is not closed, matter or energy can escape from the system.  an example of this is if you react magnesium and hydrochloric acid in a open system.  The H₂ gas is going to escape making it look like some of the mass disappeared .  in that same reaction some in an open system will also loose heat to the surrounding which will make it look like less heat was produced.

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Letter a, because when there are unlike charges they will always attract to each other.

Iassume the answer is 75 feet. inwards to outwards rather than outwards to inwards. probably be 75.hachures are stokes drawn in direction of the steepest slope. a series of short parallel lines drawn on a map to indicate topographic relief. it is actually called hachures on contour lines. the small marks in the center of the feature are called hachures.observe the picture attached