The first one, Added removed or changed. Mutation as in anything that would cause the slightest change.
The answer is A and D. At first, they are confused with the structure. All they know is the DNA is consists of A, T, C and G. After they know the number relationship, they figure out that base may be pairs(A&T, G&C). Then the X-ray helps them to get the coiled helix structure.
Guard cells are located in the leaf epidermis and pairs of guard cells surround and form stomatal pores, which regulate CO2 influx from the atmosphere into the leaves for photosynthetic carbon fixation.
Stomatal guard cells also regulate water loss of plants via transpiration to the atmosphere.
T-A-C-G-A-T because A and T pair and G and C pair
