What does it mean when a compound’s chemical formula does not include any numbers?

there is only one atom of each element.
there are only single bonds in the molecule.
all the atoms have double bonds.
the molecule does not have any bonds.

What does it mean for a cell membrane to be selectively permable

The picture shows a location in india that is very dry and arid. notice the tall mountains in the background; you can find high amounts of vegetation on the other side of the mountains. which statement is most likely true for this area?

Astudent poured a solution of bromothymol blue indicator into three test tubes. then he placed an aquatic plant in two of the test tubes, as shown below. he placed a stopper on each test tube and placed them all in the dark for 24 hours. bromothymol blue turns from blue to yellow in the presence of co2