Which sentence below is a compound-complex sentence?

a.

My friend and I went to the beach today.

c.

My friend and I went to the beach today,

so we went surfing in the ocean

b.

My friend and I went to the beach today,

but we were not able to go surfing

because it started to rain.

d.

Although my friend and I went to the

beach today, we were not able to go

surfing.