Veins .

produce red blood cells
are only found in the heart
usually carry deoxygenated blood
carry blood away from the heart

There is only one answer.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.

here is the answer

water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering. weathering is the breaking down or dissolving of rocks and minerals on earths surface. once a rock has been broken down, a process called erosion transports the bits of rock and minerals away.