My grandfather, a retired worker, often （）the past with a feeling of. longing and respect.

A．study the possibility of cloning humans
B．search for ways to modify its temperament
C．examine the reproductive system of the dog species
D．find out the differences between Missy and its clones

A．still have a 10ng way to go before reaching maturity
B．have been widely used in saving endangered species
C．provide insight into the question of nature vs. nurture
D．have proved quite adequate for the cloning of humans