第一题：我觉得选C 因为 appreciate后可接名 (代)词、动名词、名词性从句等作宾语,但不能接不定式
第二题：用法有以下三种
1）名词、代词或动名词；suggest sb doing sth
2）疑问词引导的不定式或宾语从句；suggest what to do/ what we should do
3）that引导的宾语从句:当做建议的意思讲时,后面所接的从句要用虚拟语气形式.
本人认为选c
第三题：
There are some areas( ) are underdeveloped.
A.on which B.in which C.where D.which
are underdeveloped 缺主语故不能用where（用作状语）
He was hiding behind the door( )he could see what was happening.
A.which B.from where C.from which D.where
从句不缺主宾,只缺状语