ejercicios sobre referentes

Referentes. Ejercicios

Identifica los referentes en el siguiente texto:
 
a) which (línea 2) se refiere a: ________

b) them (línea 2) se refiere a: _________

c) they (línea 4) se refiere a: __________

d) its (línea 5) se refiere a: __________

f) it (línea 15) se refiere a: __________

Roman cement: Concrete has been used as a construction material for centuries. Before 100 B.C. the Romans had developed an excellent concrete which enabled them to erect vast structures and works of engineering.

         On the slopes of Mt. Vesuvius and in extinct volcanic areas near Rome they found a light, porous volcanic rock. Its rough surface formed a good bond for cementitious material (a substance capable of acting as a cement), or mortar. The cement was prepared from a mixture of lime and a volcanic ash called pozzolana, named after the village of Pozzuoli near Mt. Vesuvius. When the pozzolana was mixed with limestone and burned, the resulting material, ground and mixed with water, gave a cement of unprecedented strength. Roman cement was a hydraulic cement, one that will harden under water.  This cement, mixed with small pieces of volcanic rock, formed a lasting concrete.

         As the Roman Empire expanded the knowledge of concrete spread throughout Europe and Asia. The quality of the concrete had to be determined by extensive trial and error. It varied from area to area, depending on the purity and composition of the available raw materials.