After walking on the zigzag path to the mountain top for a few hours, the tourists gathered under trees_____ shade and______ down to have a rest.
#93590
A
provided; sit
B
providing; sitting
C
providing; sat
D
provided; sat
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_______, the experiment will be successful. 
#88567
A
If carefully doing
B
If it done carefully
C
If carefully done
D
If doing carefully