正确答案： D
本题解析：
本句的意思是：“即使研究有了新的发展， 只有一小部分的试验可以不用动物” 。因此答案为 D。variety 和 plenty 分别表示“多样”和“大量的”，意思不合适。 amount 后只能接不可数名词，而 test 是可数名词。
_______ for a long time, most of the crops in this area died from lacking water.
The factory _______ we ’ll visit next week is not far from here.
The new laws threaten to deprive many people _______ the most elementary freedoms.
This is the first time that a woman has been ________ to the post.
He has deep love for the people and ________ loyalty to the country.
I ’ve got the order from the boss that the work _______ finished before 5 p.m. today.
I don ’t think the charge for overhauling the equipment is excessive in _______ to its size.
She ________ everything else and concentrated on the task before her.
______ he has enough money to buy the house, it doesn ’t mean he ’s going to do so