Chem Q's Help!!!

Gcse level....

when they ask you to draw the structure of an alkene does it matter the way you draw it? because its different in the cgp book to the one drawn in the mark scheme....or does it all mean the same thing?

And one more...

how are sedimentary rocks formed?

'marine organisims die/shell build up in layers/compressed to form rock' thats whatit says in the mark scheme for limestone, does that apply to sedminetary rocks??

Thanks!