This is known as parasitism.
In biology, parasitism is a relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, and is adapted structurally to this way of life.
One problem with this hypothesis is that the student didn't write about why this hypothesis might work - what is the underlying mechanism. The thing is that one might want to write the underlying mechanism or explanation in his hypothesis. Another problem with this hypothesis is that it's very hard to measure and be sure that following exposure to white chocolate, people are happier and that it's not a consequence of something else.
Getting the probability for each event can easily be done using Punnett Square (say, Hh x Hh for the couple since they are both carriers of the disease). Doing so will result to: 0.25 probability of an offspring not having the disease and is not a carrier of the gene, 0.50 probability of an offspring not having the disease and is a carrier of the gene, and a 0.25 probability of an offspring having the disease. The probability of these events apply to each offspring, and will not depend on how many children they want to have. This means each offspring has a 0.25-0.50-0.25 chance of not being a carrier, being a carrier, and having the disease, respectively.
The correct answer is (d) The number of plants and birds increases and the number of snail decreases.
Green plants are eaten by snails, snails by birds, and birds by household cat. If cat is removed from the cycle then the number of birds will increase which will decrease the number of snail in the environment. As there are large number of birds there will be a great demand of snails as a food for birds. Then there will be a decrease in the number of snails because of over eating of snails. There will be an increase in the number of garden plants because very less number of snails are left that will eat garden plants.
