Read the sentences. when bricking a building, masons must fit the bricks together. sometimes, masons slice bricks in half for a better fit. which is the best way to combine the sentences? when bricking a building, masons must fit the bricks together; sometimes they slice bricks in half for a better fit. when bricking a building: masons must fit the bricks together; sometimes they slice bricks in half for a better fit. when bricking a building, masons must fit the bricks together; sometimes they slice bricks in half for a better fit.

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.

what passage could you provide the passage.

but it is sometime nessary to hit key points or subjects   but is sometimes to much reveiw if there was a way to slim down the redone work it would improve for every body.

it would be d because the however is more proper and the comma is placed in the right spot