.will give abrainlest in the past, typhus and other diseases were more common in cities because cities were densely populated people had little immunity famine was rampant trade increased disease

Option A, cities were densely populated

Explanation:

Typhus is a disease caused by body lice. It spread like an epidemic disease when the city is overpopulated. Extreme overpopulation causes the transfer of lice from infected person's body to another person's body when they are nearby. This nearness ease the travelling of lice from body of one person to the body of another person .  

High densities in city means the number of people living in a given area is very high. Thus, large number of people living in small area ease the spread of Typhus and other disease.  

Hence, option A is correct

it has to be understood that the sea water contains a huge amount of sodium and chlorine ions and if they enter the cell of the atlantic cod fish, the cell will shrivel and the fish will die. to avoid such a condition, the fish uses active transport for the purpose of removing the ions against the gradient.

explanation:

the atlantic cod is not a maritime osmoconformer. its native application of salts is less than that of the seawater it lives in. it is continually falling water from the gills to the ocean via osmosis.

countering that, the fish is going to be swallowing seawater. but since its internal collection of salts is lower than seawater, the fish will want to go free of excess salt, and since that's going upon a gradient, then active transport, requiring energy, must be used to pump out the salt. much marine fish use the organs to filter out the salt and these fish discharge very straight urine.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.