Dutch scientists planted test crops in soil designed by NASA. The soil was similar to soil found on Mars and the Moon. The crops included different kinds of grains and vegetables. The soil contained potentially dangerous heavy metals, but the vegetables were declared safe to eat. Some of the 'space' crops contained less dangerous metal than those grown in normal potting soil.
The test crops were ________
A. plentiful
B. withered
C. bitter
D. harmless