Cactus are adapted to live in deserts and other very dry Places
a) The leaves of the cactus plant are reduced to spines. How does this help it to survive?
b)The stem is large with a space in the centre. How does this help the cactus to survive?

for a

Explanation:

if a cactus has leaves, it will need more water to survive and also there will be a higher rate of transpiration. i don't know about number 2. Try googling it.

this is math not biology xd

the correct answer is: b. pantala flavescens, because these organisms would have a high level of genetic variation due to sexual reproduction.  

because involves two organisms (parents) whose genetic material combines, sexual reproduction leads to genetic variation in new generations of offspring. the offspring is an unique organism, with new genome, more protected with the higher ability to adapted. this process is fundamental to evolution.