How much have the glaciers retreated since the end of the 19th century?

            Glaciers are very sensitive indicators of climate change. One of the methods is a measuring of the glacier mass balance. Cumulative mass balance shows how much mass it receives / loses. The unit of cumulative mass balance is m w.e. ( meters water equivalent ).  We can say that cumulative mass balance today is much lower than at the end of the 19th century ( about 60 m w.e. lower ). Therefore glaciers are losing the ice at an extensive rate.  Also the average length of all glaciers around the world have decreased for about 1,200 m in a period: 1900 - 2017.
           The glaciers retreated about 1,200 meters since the end of the 19th century.
             

Answer might be b i think

Is there a passage that comes with this question or should i know this right off the bat, because if it is, i probably have short term memory loss or college is too stressful