Question

为什么是there is a lot of exercises

Answer 1

lots of 意为“许多” 就等于a lot of 。后面可加可数名词复数，和不可数名词。eg:1.There is a lot of (或者lots of)water in the river.2.There are a lot of (或者lots of)students in my school.在这里food 是食物的意思，不可数名词，所以用is
2 there are all kinds of fruit on a small table.这句强调食物的种类，而1 there is a lot of food on a small table.强调食物的多，

Answer 2

应该要看语境,判断这里的exercise是什么意思.
exercise作为名词,
（1）译为“练习”、“操”时是可数名词.如 There are many exercises in the book.
We should do eye exercises.
（2）译为“锻炼”时是不可数名词.如 Let's take exercises
所以a lot of exercises是指“大量的体育锻炼