<span>LD
means lethal dose. LD50 is the amount of a material, administered at once to
cause the death of half (50%) of a group of test organisms. A test which shows that LD50 of a certain
chemical is 20 mg per kg means that 20 milligrams of that chemical for every 1
kilogram body weight of the test animal when administered in one dose, causes
the death of 50% of the test group.</span>
<span>This means that 20mg/per kg (each kilogram of target animal)
of the substance is enough dosage to kill 50% of the target sample population. LD50
means Lethal Dose 50. This term is critical in research in biology, medicine,
and agriculture. Mostly the substances used are lethal such as radiation,
toxins, or pathogen. </span>
<span>Well active transport absolutely moves substances against their concentration gradient (answer C), although I dislike how the other answers could probably be argued for. This seems to be a case of pick the "best" answer</span>
