Here is a new idea in agricultural weed control. Rather than trying to formulate herbicides that kill specific weeds but are harmless to grain crops, use a broad-spectrum herbicide effective against all kinds of plants and use genetic engineering specifically to make the crops impervious to the herbicide.
Which of the following, if true, is the most serious obstacle to an implementation of the new idea outlined above
A．Certain herbicides that are effective against specific weeds can inhibit the growth of certain crops as much as two years after application.
B．Research to date suggests that the nutritional properties of crop plants would not improve as a result of the genetic alterations being contemplated.
C．Most herbicides that kill only a narrow range of targets contain active ingredients that are harmful to domestic animals and wildlife.
D．Effective broad-spectrum herbicides are on the market, but their very effectiveness has so far rendered them unsuitable for agricultural weed control.
E．(E) Although genetic modification has made individual plants of grain species resistant to broad-spectrum herbicides, the seeds that such plants produce will not sprout.