In an ________ solution, red blood cells ________. 

The appearance of the Isthmus of Panama separated the Pacific and Atlantic oceans and led to:

Mules and hinnies are both the results of hybridizing between a horse and a donkey. A mule is a result of a mating between:

Which of the following compounds is most soluble in hexane (CH3CH2CH2CH2CH2CH2CH3)? 

Sexual Reproduction can speed the spread of adaptations in a population because:

In polar bears one gene, APOB, encodes a protein that binds to cholesterol. This gene has experienced strong selection because of polar bears:

Cuckoos (birds that live in Britain and mainland Europe) lay their eggs:

An evolutionary trade off in howler monkeys appears to have resulted in developmental changes in the size of their hyoid bone and the size of which organ?

Choose the correct conjugation of the verb in  parenthesis to complete the preterite sentence.  Ayer Mario ________________ a los videojuegos por cuatro horas.

¿Tienes sed? Sí, debemos visitar _________________________.