Filtrate in the proximal tube not ideal for toxicology?

Which of the following explains why a sample of the filtrate in the proximal tubule is not ideal for conducting a drug test or toxicology screen?

a. Some of the toxins and drugs do not enter the filtrate until they are secreted in the distal tubule.
b. Many of the toxins and drugs enter the bladder and become part of the urine as it is stored.
c. As soon as the toxins and drugs enter the proximal tubule, they are immediately reabsorbed.
d. Only a blood test can detect drugs and toxins because they are too large to get into the filtrate at any point along the tubule.