Match the feature with its description a: stalactites 1: cave science b: stalagmites 2: tubes without hollow centers c: speleology 3: tubes with hollow centers d: speleothems 4: cave features answer

Features matched with its description 
A: stalactites      3: tubes with hollow centers 
B: stalagmites    2: tubes without hollow centers

C: speleology     1: cave science

D: speleothems 4: cave features

Speleology is shortly defined as the scientific study of caves. Speleothems are structures formed in a cave by the deposits of minerals from water dripping from the fractures of the ceiling of the cave. Most common types are the stalactites and stalagmites.

Stalactites began as a ring of crystals which later becomes small tubes with a hollow center. Stalagmites, on the other hand, are tubes formed without hollow centers. 

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.